How do you know someone's not straight? (Attention to Bisexual, Lesbian, etc)

Well, you can start by getting to know her. She could in fact be a lesbian or bisexual and could return your feelings. It does help knowing their sexuality before you pursue a relationship to avoid hurt feelings, and if you can find a way to bring it up in an early conversation that would be advantageous.

You have to follow your gut instinct with these things but also have courage. If you don't ask her the right questions or make your interest obvious then you will get stuck not knowing how she feels and spend a lot of time in your head overanalysing.

I am a big fan of the direct approach and I find that not hiding behind fear works out best. It can be scary to admit your feelings and put yourself in a vulnerable position like that, but if you find out she doesn't reciprocate your feelings then you can stop wasting each other's time and move on.

Good luck in your romantic endeavour.