If the messiah really did return, what would he/she have to do to prove it?

He would have to resurrect himself after I throw him into a wood chipper.

When Christ returns, there will be no doubt who he is. The bible doesn't say Christ will return 'to' earth. The earth will pass away and we will meet him in the air. This is sort of a complicated study because so many believe he will come back to earth, set up a kingdom and rule a thousand years. He has already set his kingdom up and has already ruled more than a thousand years. Revelation is a 'symbolic' book, not literal. I will post scripture below and hope it helps. You asked a very good question.

When, not if, Jesus Christ returns, there will be no question or doubt in anyone's mind.

Revelation 1:7 Behold, he is coming with the clouds, and every eye will see him, even those who pierced him, and all tribes of the earth will wail on account of him. Even so. Amen.

Philippians 2:10-11 so that at the name of Jesus every knee should bow, in heaven and on earth and under the earth, (11) and every tongue confess that Jesus Christ is Lord, to the glory of God the Father.

The Truth will be made self-evident. The world is full of crackpots. The authentic Messiah wil not need to prove Himself. Truth never needs to defend itself.

Scripture says Jesus will come back with many hosts and riding on a white horse,

It says all eyes will see. It says no one will have any excuse to deny Him then. If you study miracles He did you will see that He caused many to return from being dead and so on. He was not a wimp.

He will be coming on the clouds with power and glory and will be seen by all the world at once as the bible says.

Even if you perform something undeniably supernatural, it does not prove you are God or the Messiah.

I heard he was elected president a few years ago

Fly and breathe underwater. No tricks, and it all has to be proven.

You folks equate belief with acceptance. If there were proof that Jesus or "god" actually existed, I still could not force myself to accept Christianity.

Please try to catch up!