Find out

That might be one way to find out.

Another way is to talk to her and just ask: "Are you involved with (seeing) anyone right now? Maybe we could hang out and get to know each other."

or

"Are you free to have coffee (ice cream, drink) some afternoon?"

or

"Can you study with me at the library?"

She will let you know if she is available.

(you don't say how old you are)

I'm 17 she's 16 and we are in the same class

In my day it was:

Wanna study after school today?
Can you help me with the assignment . .?
Can I walk you home?
Are you busy Friday nite for the game?
Want to meet me at ________ for a burger?

and the bestest & surest one:
Do you and your girlfriend want to go eat after school?
She won't refuse you if she can bring the friend along.

If she's hooked up with another guy, she will decline all invitations.