[Serious] Could an omnipotent and omniscient god prove that he was God?

[Angrboda]

I think the real question is what could a god do that in principle could not be faked by, say, advanced aliens? By prove I presume that you mean make so that we believe it is undeniably true that God exists. Unfortunately, many things that fall short of a god can do this. On the individual level, all it takes is a brain tumor or epilepsy. So the question turns on God being able to provide us with a judgment that is infallible. Short of changing what we are I do not imagine that this can be done. The idea that the communication channel that runs from the perceived to the perceiver is wholly dependent upon the properties of the transmitting entity is old and fraught with problems. As much of our perception is within the purview of our brains' operation, it doesn't seem possible to eliminate potential sources of error.

This of course hinges a bit on what you mean by 'prove'. If by prove you mean something less than establishing absolute certainty, then I'm not sure the question doesn't lose its value. We already have people convinced by evidence that provides less than complete certainty of the conclusion. That's a sliding scale with no well-defined stopping points.