You don't - it's against the man's code.
ask
tell her that her boyfriend cheated on her. she needs to know.
Don't tell her you love her, tell her you like her.
if i where you i would tell the girl that likes that boy that he has a girlfriend.that is what i would do.
Tell you her your feelings about her
You should tell the other girl and both break up with your boyfriend.
Tell him the truth, say you like girls.
There is only one way to tell: you have ask her.
Just tell him gently....."listen, I have a boyfriend. Just saying"
You ask
If your boyfriend is a player, he will probably make excuses for spending time with you. If he is a player you should talk to him about it and tell him he was wrong, but dont get mad that will make things worse. You should also brake up with him. Other ways you know he's a player are: avoids you when you when you get close to you sometimes. With another girl when he is not with you. Etc.
you dont tell her, let her find it out for herself,