Usually a good indicator of whether or not you should approach a girl with another guy is her body language. If she is leaning into him, looking enthusiastic and upbeat, etc, then she is most likely attracted to him and you're better staying off.
On the other hand, sometimes I'll see couples walking around and the girl looks bored, her body language is showing disinterest in the guy... Usually these same girls will hold eye contact with me if I catch their eye. If they hold eye contact with you for a good time, then the other guy is probably an AFC / you sparked her interest. This all usually happens when the other guy is looking away to a different direction.
At this point you'll both "know" that you want her and she wants you
, so she's sorta even hoping that you'll approach her somehow someway.
Now the challenge is that even if she is interested in you, she would not usually be too keen on giving you her # right in front of the other guy, even if she is NOT interested in him (it's all about avoiding looking like a sl*t or easy, women will do this at all costs).
Your best bet? Catch her when the guy steps away or she walks off to somewhere else by herself. What if you got no choice and must talk to her with the guy there? Then AVOID asking them if they are in a relationship; don't say stuff like (Is this your brother/friend/boyfriend?). Just keep the focus of the attention off their connection/relation. Chat up the girl with a little fluff talk. Once in a while the guy might start talking, to him just say stuff like "hey are those Levis Jeans? Cool. I got the same ones!" Then turn back to the chick and continue talking. Don't make it look too seductive/romantic/sexual, but with the girl make it look like a "buddies connection." Here you're talking about similar interests, places to go to, things to do. Then just finish off with "hey look i gotta get going, gimme your # and we'll stay in touch" She'll prefer you not to lean too far into romantic communication because the other guy is there (trying not to be a sl*t thing, once again).
THen later on when you contact her when the other guy isn't there, you can kick your seduction up a notch :rockon:
If you do this routine right, she will "get" what's going on and so will you. Maybe even the other guy, but it will all be so subtle and smooth that the other guy cannot implicate anyone for anything without looking like an *******. And if he does, this makes him even more AFC, only making your odds better :yes: