How do you when God is talking to you ?Or when the devil is talking to you ,Or if it's just your own thought coming through your head ?

If you're really hearing voices in your head it's due to a chemical imbalance of some kind and/or a misfiring of sections of the brain.

A study of the Bible will help a person to know the voice of their master, God. Here's the example Jesus gave:

John 10:4,5

When he has brought all his own out, he goes ahead of them, and the sheep follow him, because they know his voice.5 They will by no means follow a stranger but will flee from him, because they do not know the voice of strangers.”

When we understand the thoughts and actions of God we will know his "voice" or his ways or direction in our life. How do we do that?

John 8: 31- 32 : Then Jesus went on to say to the Jews who had believed him: “If you remain in my word, you are really my disciples,32 and you will know the truth, and the truth will set you free.”

Jesus said we need to remain in his word. Jesus was the perfect reflection of his Father. We need to study the bible, because that is the way that God reveals himself to us.

"Who among you is wise and understanding? Let him show by his good behavior his deeds in the gentleness of wisdom. But if you have bitter jealousy and selfish ambition in your heart, do not be arrogant and so lie against the truth. This wisdom is not that which comes down from above, but is earthly, natural, demonic. For where jealousy and selfish ambition exist, there is disorder and every evil thing. But the wisdom from above is first pure, then peaceable, gentle, reasonable, full of mercy and good fruits, unwavering, without hypocrisy. And the seed whose fruit is righteousness is sown in peace by those who make peace." (James 3:13-18)

First of all, YOU are always 'in your head' and those spiritual influences, (good or evil), never remove you from the equation. We can learn to be a good listener, responding to GOD's influence with 'eupeithes,' (Greek: literally, 'trusting in the best,' translated 'reasonable' in the above verses). When we are being influenced by evil, we respond by feeling very resistant instead, lacking trust in the motives or intents of all external influences. We also become driven by a viewpoint of 'bitter competition, jealousy and selfish ambition.'

When GOD is influencing our thoughts, we forget any comparisons with others, we stop seeing ourselves as part of competing groups, we feel no desire to 'win' anything at others' expense, both our own needs and those of others seem equally important, we don't see any newly arriving opportunities, power or genius as a tool for personal advancement or advantage, we feel confident in our 'outer' person with no need to hide behind masks.

If any of those qualities are absent, you are likely being influenced by evil. It is NEVER just you 'in your head.' However, the degree of influence does change and your response to it determines how strongly and clearly which influence is guiding you.

Only in very rare and sacred moments does GOD 'take over' and only with our full consent beforehand and during under specific special inspiration and according to specific rules within an art called 'prophecy.' It is comparable to agreeing to accept the flow of genius from the deepest well of human art there is. Even then, there are many 'filters' between us and GOD or us and the sources of evil behind that inspired art. You shouldn't see your experiences as 'GOD, me, or the devil' speaking. Even that smacks of evil influence, ridiculously attempting to pit GOD, humans and the devil in some kind of jealous competition. It doesn't work that way at all.

We can fully 'purge' evil away as we embrace full intimacy with GOD, but we do this by learning to fully differentiate good from evil through 'repetitive practice,' (Greek: 'hexis,' Hebrew 5:14). That is a process since evil frequently masquerades as 'goodness, 'enlightenment,' 'progress,' and other seemingly 'noble' things, (2 Corinthians 11:14-15).

God doesn't talk to us directly like he did back in the old days. He always showed his hand. Today, he's not letting people be spoken to directly (at least not anyone in the world) by him nor his angels in heaven until his appointed time comes. To be clear, I think there's a way to see God's hand better. Just do what he asks of you and don't fail at it. I don't think anyone can hear God's voice until some time after the great day comes.

the Bible tells you to "test the spirits"

Isaiah 8:20 "If they speak not according to THIS WORD it is because there is no light in them."

It's only imaginary. God and the devil don't exist

It's literally just your inner monologue. If you pretend your inner monologue is a deity, you're on the fast track to a special place with special vests, full of special people, all convinced they're the next jesus. At least until their meds kick in. Then, they're far more like jesus. They turn into zombies.

The devil can't reach me.

... One is proven to occur.... The other 2 participants aren't even proven to exist. ....

Onedrop, prove it