You can start a conversation about movies, then start gushing about some hot actress.
Then be like, “I’m not gay, but I’d totally be gay for her!” or something equally circumspect. Then just see how she reacts. If she seems game, you can start probing her sexuality a little more openly. Has she ever thought about kissing a girl, who could tempt her over to the dark side, stuff like that.
You’ll get way more information this way than if you just out and asked her if she was gay. “Gay” generally doesn’t include ‘curious’.
If the conversation drifts to other topics, don’t try to bring it back there or you’ll risk looking like you have an agenda. Wait a little while.
I’ve had a lot of gay guys hit on me when I was a lot younger. The only ones that got anywhere were patient and let me set the pace. I’m not even remotely gay, but I’ve had a few experiences, enough to where I’m completely sure that I’m 100% straight.