After Bryan Mills observes the Albanians all have beards and have the star and moon tattoo on their right hands (as per Kim's description on the phone), how does he know to give the "Good Luck" note to the correct "Marko".
In the scene, you can see Bryan talking to all of the different people, getting them to answer trivial questions. When one of them stays quiet, he gives him the note to read based on his elimination of the other "Marko"s responses where they answered.
As the "correct" Marko is the only one who hasn't spoken at that point, he plays a slight trick in giving the note to him to see what he says. It's a gamble that pays off as it allows him to then ascertain the correct one, and do do what he does from there.