There could certainly be a being who believes that he is God, but which is not. If this being has been created by another being, then he is not a final cause, and thus, he is not God. But yeah, “how God knows that he knows” is a puzzling question, I have to admit it, maybe it is one of these things our human minds cannot compreehend (which I’m sure is a answer that will not make you happy, haha). But I think that we can at least agree that God being omniscient within the universe is perfectly consistent, if God is the force which sustains it, it could as well have perfect access to everything in the universe at all times (block universe, maybe?).
I don’t see how that is incoherent with the idea of a God which is the creator and sustainer of the universe, and omnipotent and omniscient at least in within the boundaries of the universe/existence (God is able to do anything logically possible, and is aware of everything that happens in existence), and if God is already perfect, he doesn’t need to change his mind/be unpredictable, perfection is actually pretty simple in that sense of being predictable, Richard Swinburne actually uses that as a central argument to dispute the claim of Richard Dawkins that using God as an explanation makes things more complicated instead of more simple.