How do you tell if a girl likes you back?

You could try some friendly affection and see how she reacts. Try giving her a kiss on the cheek. If she goes "Ew!" and pushes you away then she either doesn't like you in that way, or she isn't ready for that kind of relationship.

You could also try subtly asking her how she feels about homosexuality and lesbianism and find out if she's ok with it or if she's repelled by the idea. If she's ok with it you could ask her if she's ever had a crush on another girl.

If she's cool then maybe you could just ask her if she likes you.