Oh boy... DJ bible time. Why do you even CARE if she's with someone? You could be having a great convo. She's really digging you and then you have to go remind her that she has a BF.
She may be deciding that you are better than her current BF and decide to switch over to you, but if you ask and she tells you yes that instantly changes the dynamic. She now verbally stated her identity as a GF of another person and to go out with you would go against her stated identity. This may seem like a small point but twice I charged up...started dating these girls and only later found out that they had dumped their BF to be with me.
Anyway you WILL find out if she's with someone...just go for it. If she's serious about the other guy she will tell you flat out, but if the answer to your approach is yes then you are in and it doesn't matter if she was "with someone"