A girl often will stay with a guy even when she's attracted to you. For starters, she might not want to test the waters if she's unsure of your attraction to her. She might be confused by her feelings for you. If you suspect a girl with a boyfriend likes you, yet you can't be sure, you need to learn the subtle clues a girl with a boyfriend will give you if she's genuinely interested.
Pay attention to body language. If, while sitting or standing next to her boyfriend, she turns away from him and toward you on more than one occasion, she might be sending you signals. If she touches you often during conversation and you notice that she doesn't do this with others, it's a sign of interest. Another tell-tale sign is in the tilt of her head. Does she tilt her chin toward you when you speak? Does she repeatedly nod in affirmation to your comments? These are all clues that she is probably unaware of.
It's in the eyes. The difference between body language and eye contact is that a woman has more control over her choices of where she allows her gaze to fall. If she never looks away when you speak, not even for the natural act of looking at her glass when she reaches for it or acknowledging someone else in the room, she's likely attracted to you. Another trick she might use is giving you a smoldering look, a long gaze with a hint of a smirk that leaves you feeling uncomfortable, but mesmerized at the same time.
She calls you before she calls her boyfriend. Nothing gives away a girl's true feelings like calling you in a crisis instead of her boyfriend. If she calls you to fix her flat, repair her sink or--oops--it wasn't leaking after all? She's letting you know there's an open door.
Pay attention to the conversation. When she steers conversations to include plans that leave her boyfriend out of the picture, you can bet she's putting out feelers. Conversations that include jokes such as, "If only we had met before I hooked up with Joe ..." or "You and I are so much alike..." she is hinting that she likes you.