I think I’d say: If you’re God and you can prove it, you don’t need me to answer the question.
As I used public transportation for years when I was in college, I’ve had people come up to me claiming to be God and saying they could prove it. Obviously, they couldn’t. If God really wanted to convince me, He’d know not to approach me with that claim. Then again, just about anything He could do would make me assume I was hallucinating and then I’d have to doubt again.
So…well. I guess He couldn’t prove it. In which case, the question I’d ask would probably be, “Could you go away now?”
I guess it would have to be something quantifiable, so I would ask him to make a few million Euros appear in my bank account.
Now that won’t prove that he/she is a god, but I would be willing to call them god.
Donald Trump, do you hear me?
Bill Gates, anybody?
Probably a big enough act of “magic” would suffice to convince me that he/she/it was powerful enough that I’d consider them a god. Along the lines of whisking me to the other side of the world, turning the stars on and off, or maybe even just convincingly levitating a bus.
Can’t really imagine a test that would prove an entity was the “only” god.
And he would all-knowingly smile and proceed to do what?