dude take my advice
ask her out on the movies and ask whats up
talk about stuff your future, talents, funny stuff, news
remember (unlike matt's advice)
if you see her with other people ask her who's her new friend
if you know the guy don't over react get acquainted with him
see if he's worthy of being her lover
don't tell him about your gf and you
ask him if he has a gf if he tells you his gf is your gf meet up with both of them (plan it as a coincidence) and know the truth
now if she says she has another tell her "if you like him its okay he's a good man"
if she asks you why just say "because i love you thats why I'm letting you go"
tip:make some friendly jokes about it
don't give her consequences , dont threaten her and see her as a friend