How can I tell if the girl I like is a lesbian too?

She has short pixie cut hair, but she dresses girly and wears make up. I like feminine girls hence the reason why I am crushing on her. She graduated a year after me, but we go to the same college now and have a class together. We didn't really know each other in HS I knew of her and had a crush on her in middle school. In high school I didn't see her much at all. My sister is a senior in hs right now and she had a chemistry class and was this girls lab partner. My sister said she is very flirty, but as far as she knows didn't date anyone in highschool. She doesn't have facebook so I can't see if she has a preference from her profile. I don't wanna straight up ask her because that's just awkward