If I never catch a girl looking at me, is she not interested?

Like if I'm walking around, talking to other people. I've always wondered if she just isn't interested or if it's her personality since I've never seen her look around really and she's the type of girl who doesn't talk to anyone or go up to anyone unless they go to her. I've made the effort several times to go up to her, but I never catch her looking at me otherwise.
If I never catch a girl looking at me, is she not interested?
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