You don't. >_>;;;
Ask the girl.
ask her or get somone to ask her
ask her
ask
"be ma boyfriend"
Ask your boyfriend. If you don't trust his answer he shouldn't be your boyfriend.
the smartest thing to do is ask.
If I was the girlfriend, and I know what's going on, I will talk to this girl and ask her what she is trying to do. I will ask her if she knows that the boy she is trying to flirt has a girlfriend. I will be civilize and calm and be professional about talking to the other girl. I will also tell her how will she feel if it was her boyfriend that's being flirt by another girl. Then I will ask nicely to stop flirting with my boyfriend.
You ask
The most appropriate way is simply to ask her.
Ask other people to ask that girl if she has a boyfriend. Just remind them not to mention that you wanted to know.
Well, if you are a girl whose boyfriend, who also has a boyfriend, you ask him is he is bisexual or gay. Whatever his answer, only you will know if you want to continue as his girlfriend, his friend, or if you want to move on.