There's no clear way to tell, man. There are girls at my uni that have bfs at home and will sleep with other boys, but at the same time there are girls that will be with their man for the rest of their lives.

If you think she's classy, you respect her, she's beautiful to you then she's probably staying with her bf
if she's trashy, flirty, hot, then she'll fuck you. but she's probably not what you want anyways.

From what I've seen anyways.
But seriously you should just go for someone else. No sense wasting your time on one girl when there are plenty of others just like her and better.