Well, what you can do is once you get to know her, ask her if she has facebook and facebook usually says Interested In: Men or Women!

You can also go out with her (just as friends) and ask her about past relationships...If she says, "I dated this guy " or "I dated this girl..." then you know what she likes, I guess?

Even if she's straight, lesbian, or bi-sexual, you can always ask her to go out.