Switches on spy mode.There are two really very very easy method.For that,Go to his place and ask himOr simply ask him on messengerDone. You have got your answer!Ok, jokes apart, it is a question of maths, Permutation.First check for mutual friends. You can only check for them.Now check how many are active.You have got a small set, usually 3u20135 people.Now, go to their profile and try to check who he or she had recent conversation with, probably in comments.You are ready to make a guess! Congrats.Bang your head against the wall.If you aren't convinced, better not interfere in other's privacy.Back to normal homo sapiens mode.