How do know if girl wants to date you? And what if she has on again off again boyfriend?

How do you know? You ask her out to something specific.

What if she has an on/off boyfriend? Ask her out when he's off, then do a better job not being a jerk than he does.

I think you can ask her to go to a movie or something and see how she responds.
I'd watch that on again off again boyfriend though that can be a heartbreaker
If you know what I mean ; )