Was she just looking at me or checking me out?

I was on my phone at the gym, I looked up and this girl had her head slightly turned at me with a gaze look on her face looking right at me walking up the stairs for like 3-4 sec. We made eye contact and she looked away hastily and like embarrassed and I looked away as well. I looked back up like 3 sec later and again she looked at me we made eye contact and she looked away hastily again.

Two weeks later I saw her I introduced myself and apologized and she was like "no worries! No worries!" smiling and and got her name and told her mine. Later she was again coming up the stairs and glanced at me for a few seconds.

What is the difference between just looking and checking you out?
Was she just looking at me or checking me out?
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