What kind of eye contact am I looking for at bars?

I don't go clubbing or bar hopping much, that's not how I usually meet women. But I'm starting to do it more so that I get more comfortable in those environments.

Tonight I had a few looks from some girls at the bar I think... anyway one girl we made eye contact maybe 3 or 4 times and it lasted a few seconds. The other girl, she was talking with her friends and laughing and having a good time, but 2 or 3 times our eyes met for like a second and she would look back at her friends.

The first one I think she was checking me out or looking at me because of how long she looked and how often, but the second girl? She looked several times but it was only for a second, no more. Was that just a coincidence that our eyes met like that a few times? What kind of signals do you look for in bars/clubs to know a girl wants you to approach her?
What kind of eye contact am I looking for at bars?
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