You are visited by someone claiming to be you from the future, how do you make them prove they’re you?

As them the passphrase.

You gotta’ have a future-self / transporter clone / alternate timeline ally passphrase pre-decided.

Depends how far along from the future, because I doubt I’ll live that long to begin with.

Probably ask them what our time loop code is. I guess it could also be a time travelling future version of myself code.

I’ve always done this thing where I take “snapshots” of my life, when things seem very surreal. I have 5 snapshots in total. Describe two of them.

Me: “What number am I thinking of?”

Future me: “Sixty-nine, dude.”

Me: “WHOAH!”