How do you find out if another girl is bi/lez or straight?

I have this girl I really like in my AP BIO cl and I want to ask her out but, being a girl I don't know if she likes girls and I don't want to just let her know I like girls if she doesn't. That would make my school year hell the one girl I like hating me. So please any suggestions would be greatly appreciated! No Jerks or Pervs!