How do i find out if a girl is gay?

Make out with her. No, ha ha, that'd go pretty badly if she weren't.

Ask her.
Tell her you're gay (or bi) and she might come out to you.
Confuse her to the point of outing herself.
"Are you gay?"
"No"
"So you're gay?"
"I just said no"
"So you're straight?"
"Yes"
"What's it like to have sex with a boy?"
"I don't know, I'm a virgin"
"So you're gay?"
"No"
"Then why haven't you had sex with a boy?"
"I'm not ready yet"
"So you're gay, then"
"Yes.... No!!!... WTF?!?!?!?"

start talking about boys and watch her expression.
or just ask her. if she asks why you asked just say it was a rumor.

ask her if she has a bf. if she has ever had one? if she is interested in a guy?

Ask her.

Don't get all sneekey.

Just ask.

its all about eye contact if she cant hold eye contact with you for very long chances are shes not a lesbian but ont he other hand she might be scared and afraid of comming out. watch how she acts around some of her friends which she would not ave a sexual attraction to and then see how she acts with you if she acts differently ( in a good way) then you may be in with a chance.
good luck hope everything works out just fine for you.
xxxx