You should always have a question ready to ask in the event that a mysterious stranger emerges from the shadows and claims to be you, returned from the future. This needs to be something you remember clearly but have never told anyone. For example, mine is the memory of a shower curtain with roller skating bananas from my house in second grade (details altered to protect my time traveling authentication credentials). Theoretically, your future self should be able to volunteer this information without any prodding from you.

My wife and I have also exchanged such secrets in case one of us is hurled back in time to high school or elementary school and needs to prove to the other that they came back from a future in which we were married.

Bonus Round: What if your future self confirmed his identity beyond any possible doubt then commanded you to attack your boss or burn down your house "in the next 47 minutes, or you will regret it bitterly for the rest of your life." Would you do it, even if there was no conceivable justification? How could you not, knowing that you could only have your best interests at heart? This would make a good short story.