Like most guys it took me ages to realize when women were actually approaching/hitting on me (instead of just being friendly/social). If she smiles way too big for way too long (to the point where it almost seems creepy) that's what I call the "alien smile" and means she is attracted to you and isn't just being friendly.

This is often mixed with her thinking of a random excuse to talk to you then holding the smile for an unnaturally long amount of time afterwards.