How do I know if a girl that I like likes me back?

Okay, so there is this girl in my math class who I think is very attractive. We tease each other all the time. She will sometimes touch me playfully (punch on the arm, etc.) and I'm not sure if that is her way of flirting or if she just does that to everyone. Part of me wants to just go for it and tell her how I feel, but then I worry that if she doesn't then she won't want to be friends anymore. I could really use some advice on this situation.

How do I know if a girl that I like likes me back?
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