I don't think I ever really stare at anybody.
I guess what I'm saying is: I don't think there's any way of telling the meaning behind someone's glance or look. Ok, somebody looks at you. That could be either "Ooh, he/she's sexy!" or "hey, there's a person." It's pretty much pointless to think about. I get eye contact and looks from hot girls, ugly girls, guys, grandmas, plumbers, etc. This is because I'm another human being.
Now, I know there's exceptions where a person is blatantly checking another out or staring as you said, but still.
I was with a buddy a few weeks ago. A girl walked passed and held eye contact with me for a few seconds and slightly turned her head to hold it as she went by. My buddy said something like "Hey, she was checking you out!" Still, I didn't think anything of it because it didn't seem very unusual. Maybe I'm just off.