I like the ploy of asking an apparent Supreme Being (SB), "What would be the best question to ask you?" but only if you can also make sure that the SB answers that question. How frustrating would it be if SB responded, "You should ask me, 'What is the meaning of life?'" and then laughed at you as you realized you'd used up your one and only question! But I don't see how this question would help you determine if the SB was really God or whether your vision was real or a hallucination, dream, or matrix-like experience induced by a powerful but not supreme being. Heck, you could ask me what question you should ask, and I could give you a good answer. (Ask what is the meaning of life!)
So, what would constitute proof that your vision of an SB was genuine? Nothing, if your standards are set at Descartes' level of proof--you could be dreaming or in a matrix and never be able to tell, no matter what the SB said or did. But you could use a more reasonable standard, like best explanation for the observed phenomena. So, you could ask the SB to produce effects that would be best explained by your having asked such a being to produce them--e.g., "Tomorrow, allow me to fly unassisted for an hour and cure malaria and the flu." When you fly and read headlines about the diseases being eradicated, that seems like good evidence that the SB you met is pretty supreme. If SB is not willing to produce such evidence, you should taunt him/her/it: "What's the matter, you're not supreme enough to do it? Come on, show me a sign!"