We know what the boyfriend bomb is, but sometimes you talk to a girl you like who won't tell you she has a boyfriend so as to waste your time. So I'm thinking you want to ask her things that might get her to say it. For example "does your boyfriend say this?", "who did you go on holiday with?" is usually a good one, "you guys are together right?" etc. etc. Life is short. You simple don't want to waste your time talking or crushing on a girl who is not available.