Continue the flirting. Show interest. Does she know that you know she's dating this guy? If she thinks you're clueless about her dating life, ask her out now and pretend you don't know. She might say something to the affect that she's seeing someone, but it'll get the idea pretty well set in her head that you want to ask her out. Then wait a few days after that guy dumps her, and ask her innocently if she's still seeing that guy or if you two can go out now.

Sneaky, but it should work and let her know that you're willing to date her.

Alexi