How do I know it is okay to approach a girl at a bar/party?

So I remember a time I was at a party and like I couldn't really tell if this woman was checking me out or not cause she kept glancing at me. (Room was well lit) so I decided to make my way over and try to talk but as I did she ignored me and walk away.
How do I know it is okay to approach a girl at a bar/party?
How do I know it is okay to approach a girl at a bar/party?
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