How do I know if A girl likes me if im one myself?

How do I know if this girl likes me?Because im one myslef and Ive been crushing on her for 2 years now I know a person she likes and theres one more person though but she wouldnt tell me and im kinda scared because our we have a really good friendship and I cant stop staring at her in class and when she looks up from her work and sees me starin at her she asks me what and smiles a bit then I just say nthin and afraid to tell her and she knows I like her.

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