Ever walk past someone you find attractive?

When I'm walking, I usually always keep my head up and look at the person I'm passing. I notice that girls sometimes keep looking forward or turn their head away. When I smile and say hello they either just smile. Or some will say "hi". How do I know if the girl is happy and thinking "OH MY GOD HE JUST SPOKE TO ME!" or is disgusted and is thinking "oh my god I can't believe that creep just spoke to me"

How do I know in that split second whether I have a chance or not?

What is the best way to approach a girl that I am just walking past?
Ever walk past someone you find attractive?
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