The best way to confirm is to ask something that only you would know the answer to, that cannot be found in any records (including speaking aloud because of the risk of recording), that is extremely unlikely you'd ever share (to rule out the possibility of it being someone else from the future), and that you can reasonably expect you would remember at any point in the future, barring any significant damage to your memory. Ideally, something insignificant that you have nonetheless remembered for an unreasonable amount of time. Given all that, my question would be: "What is the answer to the question that I would use to confirm your identity as future me?"