Well, I'd be a very different person had I never knew you, but I suppose you could tell me about all of the events we went to, that presumably I still went to but you weren't present at (in this alternate reality). Like my PDF experiences or gaming in Buffalo or knowing about my travels in NZ and China...
I'm interested enough in the fantasy that I probably wouldn't be that hard to convince..
I can tell you this much: I wouldn't start the conversation with "You're tall." ;)
I think with you I'd honestly start with the truth and work from there. "Earl, I know you're a super-secret spy, so now that that's out of the way you need to believe that I'm from another reality."
I think all you'd have to do is mention me being a 10 year old girl with pig tails. That'd probably get me to go... 0.o
Tomatoes. Just tomatoes :)
It might be safer to just pretend you're a gifted psychic, get to know your former friends again, and if you make any weird slips (stuff that you know that you shouldn't know in that reality), chalk it up to the ESP.