If a girl doesn't notice you how can you tell she's interested or not?

I have a problem I only approach women that I catch looking at me multipe times because that lets me know they at least find me attractive but if I see an attractive girl during lunch at school and she doesn't even look at me I don't have the balls to approach because I'm afraid. So I'm asking if a girl doesn't notice you is there still a chance she can like you. I get confused over physical attraction and just meeting someone and getting to like them along the way it confuses me. I always think if she doesn't look at me she's not interested in me.
If a girl doesn't notice you how can you tell she's interested or not?
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