Ramadan, how do you know that Muhammad was telling the truth?

He wasn't; a lot of the time. He actually lied when he claimed to be a prophet / messenger.

I know because I do

As Prophet Muhammad (S) telling about the story of the Cave being asked by The People of the Books during his time, regarding a group of young men who were a true believers in Prophet Jesus(A) as the true Messenger of GOD, then they were threatened by the King during their time to crucified them all .So the group of this people then they fled to hide in a cave and GOD cause them to sleep there for 309 years .

People then start guessing their numbers with a dog,some say four of them the dog was the 5th,another say five of them the dog was the 6th and other say six of them the dog was the 7th.

This story being related in details in Al Quran at Chapter Al Kahfi or The People of the Cave Surah No.18 which is equal the addition of (5+6+7)

The Prophet said GOD knows best their exact numbers and GOD promises to return them together with Jesus on his second returning to earth before the last hour comes.

Childhood indoctrination, mostly.

by the facts set out in the Quran, which some could have only come from the creater himself.. Mohammed was illiterate.. He couldnt read nor write.. so He certainly didnt write the Quran..

reading the Quran

Why can't Accepting Reality answer a question without referring to the book,..can't he just speak with his own rational thinking

nothing because islam itself is illogical very very illogical

Because Muhammad's words were guided by Allah so his words were in line with the Quran and his words was tellng the truth

Why pick on Muslims specifically, when every so-called "prophet" or "messiah" or "holy book" has made equally ludicrous claims?

All the evidences, prophecies, signs of Allah aza wa jal, his good character, etc. There is a hadith that explains it well if you care to read, none of his claims were ludicrous at all (this is your opinion entirely), everything as per Islam is logical once you accept that Allah, Lord of all things, is capable of anything (this simple thought). Hadith below:

Narrated 'Abdullah bin 'Abbas:

Abu Sufyan bin Harb informed me that Heraclius had sent a messenger to him while he had been accompanying a caravan from Quraish. They were merchants doing business in Sham (Syria, Palestine, Lebanon and Jordan), at the time when Allah's Messenger (ﷺ) had truce with Abu Sufyan and Quraish infidels. So Abu Sufyan and his companions went to Heraclius at Ilya (Jerusalem). Heraclius called them in the court and he had all the senior Roman dignitaries around him. He called for his translator who, translating Heraclius's question said to them, "Who amongst you is closely related to that man who claims to be a Prophet?" Abu Sufyan replied, "I am the nearest relative to him (amongst the group)."

Heraclius said, "Bring him (Abu Sufyan) close to me and make his companions stand behind him." Abu Sufyan added, Heraclius told his translator to tell my companions that he wanted to put some questions to me regarding that man (The Prophet) and that if I told a lie they (my companions) should contradict me." Abu Sufyan added, "By Allah! Had I not been afraid of my companions labeling me a liar, I would not have spoken the truth about the Prophet. The first question he asked me about him was:

'What is his family status amongst you?'

I replied, 'He belongs to a good (noble) family amongst us.'

Heraclius further asked, 'Has anybody amongst you ever claimed the same (i.e. to be a Prophet) before him?'

I replied, 'No.'

He said, 'Was anybody amongst his ancestors a king?'

I replied, 'No.'

Heraclius asked, 'Do the nobles or the poor follow him?'

I replied, 'It is the poor who follow him.'

He said, 'Are his followers increasing decreasing (day by day)?'

I replied, 'They are increasing.'

He then asked, 'Does anybody amongst those who embrace his religion become displeased and renounce the religion afterwards?'

I replied, 'No.'

Heraclius said, 'Have you ever accused him of telling lies before his claim (to be a Prophet)?'

I replied, 'No. '

Heraclius said, 'Does he break his promises?'

I replied, 'No. We are at truce with him but we do not know what he will do in it.' I could not find opportunity to say anything against him except that.

Heraclius asked, 'Have you ever had a war with him?'

I replied, 'Yes.'

Then he said, 'What was the outcome of the battles?'

I replied, 'Sometimes he was victorious and sometimes we.'

Heraclius said, 'What does he order you to do?'

I said, 'He tells us to worship Allah and Allah alone and not to worship anything along with Him, and to renounce all that our ancestors had said. He orders us to pray, to speak the truth, to be chaste and to keep good relations with our Kith and kin.'

Heraclius asked the translator to convey to me the following, I asked you about his family and your reply was that he belonged to a very noble family. In fact all the Apostles come from noble families amongst their respective peoples. I questioned you whether anybody else amongst you claimed such a thing, your reply was in the negative. If the answer had been in the affirmative, I would have thought that this man was following the previous man's statement. Then I asked you whether anyone of his ancestors was a king. Your reply was in the negative, and if it had been in the affirmative, I would have thought that this man wanted to take back his ancestral kingdom.

I further asked whether he was ever accused of telling lies before he said what he said, and your reply was in the negative. So I wondered how a person who does not tell a lie about others could ever tell a lie about Allah. I, then asked you whether the rich people followed him or the poor. You replied that it was the poor who followed him. And in fact all the Apostle have been followed by this very class of people. Then I asked you whether his followers were increasing or decreasing. You replied that they were increasing, and in fact this is the way of true faith, till it is complete in all respects. I further asked you whether there was anybody, who, after embracing his religion, became displeased and discarded his religion. Your reply was in the negative, and in fact this is (the sign of) true faith, when its delight enters the hearts and mixes with them completely. I asked you whether he had ever betrayed. You replied in the negative and likewise the Apostles never betray. Then I asked you what he ordered you to do. You replied that he ordered you to worship Allah and Allah alone and not to worship any thing along with Him and forbade you to worship idols and ordered you to pray, to speak the truth and to be chaste. If what you have said is true, he will very soon occupy this place underneath my feet and I knew it (from the scriptures) that he was going to appear but I did not know that he would be from you, and if I could reach him definitely, I would go immediately to meet him and if I were with him, I would certainly wash his feet.' Heraclius then asked for the letter addressed by Allah's Apostle

which was delivered by Dihya to the Governor of Busra, who forwarded it to Heraclius to read. The contents of the letter were as follows: "In the name of Allah the Beneficent, the Merciful (This letter is) from Muhammad the slave of Allah and His Apostle to Heraclius the ruler of Byzantine. Peace be upon him, who follows the right path. Furthermore I invite you to Islam, and if you become a Muslim you will be safe, and Allah will double your reward, and if you reject this invitation of Islam you will be committing a sin by misguiding your Arisiyin (peasants). (And I recite to you Allah's Statement:)

'O people of the scripture! Come to a word common to you and us that we worship none but Allah and that we associate nothing in worship with Him, and that none of us shall take others as Lords beside Allah. Then, if they turn away, say: Bear witness that we are Muslims (those who have surrendered to Allah).' (3:64).

Abu Sufyan then added, "When Heraclius had finished his speech and had read the letter, there was a great hue and cry in the Royal Court. So we were turned out of the court. I told my companions that the question of Ibn-Abi-Kabsha) (the Prophet (ﷺ) Muhammad) has become so prominent that even the King of Bani Al-Asfar (Byzantine) is afraid of him. Then I started to become sure that he (the Prophet) would be the conqueror in the near future till I embraced Islam (i.e. Allah guided me to it)."

The sub narrator adds, "Ibn An-Natur was the Governor of llya' (Jerusalem) and Heraclius was the head of the Christians of Sham. Ibn An-Natur narrates that once while Heraclius was visiting ilya' (Jerusalem), he got up in the morning with a sad mood. Some of his priests asked him why he was in that mood? Heraclius was a foreteller and an astrologer. He replied, 'At night when I looked at the stars, I saw that the leader of those who practice circumcision had appeared (become the conqueror). Who are they who practice circumcision?' The people replied, 'Except the Jews nobody practices circumcision, so you should not be afraid of them (Jews).

'Just Issue orders to kill every Jew present in the country.'

While they were discussing it, a messenger sent by the king of Ghassan to convey the news of Allah's Messenger (ﷺ) to Heraclius was brought in. Having heard the news, he (Heraclius) ordered the people to go and see whether the messenger of Ghassan was circumcised. The people, after seeing him, told Heraclius that he was circumcised. Heraclius then asked him about the Arabs. The messenger replied, 'Arabs also practice circumcision.'

(After hearing that) Heraclius remarked that sovereignty of the 'Arabs had appeared. Heraclius then wrote a letter to his friend in Rome who was as good as Heraclius in knowledge. Heraclius then left for Homs. (a town in Syrian and stayed there till he received the reply of his letter from his friend who agreed with him in his opinion about the emergence of the Prophet (ﷺ) and the fact that he was a Prophet. On that Heraclius invited all the heads of the Byzantines to assemble in his palace at Homs. When they assembled, he ordered that all the doors of his palace be closed. Then he came out and said, 'O Byzantines! If success is your desire and if you seek right guidance and want your empire to remain then give a pledge of allegiance to this Prophet (i.e. embrace Islam).'

(On hearing the views of Heraclius) the people ran towards the gates of the palace like onagers but found the doors closed. Heraclius realized their hatred towards Islam and when he lost the hope of their embracing Islam, he ordered that they should be brought back in audience.

(When they returned) he said, 'What already said was just to test the strength of your conviction and I have seen it.' The people prostrated before him and became pleased with him, and this was the end of Heraclius's story (in connection with his faith).