Dear Riyad Rover

You use a known email (or mudcat PM) to ask the user if the Fakebook ident is him her or it. Doh!

Or you wait until you meet the person (for example in the pool of the hotel Triton, Torremolinos, doing the breaststroke) and ask.

Or you message on Fakebook to ask for a detail that you hope only you and the "Friend"'s real identity knows.

Lots of ways.