I will occasionally look when I pass a cute guy on the street, but I almost immediately chastise myself for it. Like PopBoy, I have self-esteem issues, so the conversation with myself will go something like this: "Ooh, he's cute -- no, stop looking. He'd never be interested in you. Why bother?"
I can only think of three times when I've noticed a guy checking me out. One was on the subway about eight months ago, when I got up to stand near the door as the train was leaving the stop before mine. That was actually an all-out staring contest - when I got up, he looked right at me, which caught my attention, and we spent the two minutes between stations looking right into each other's eyes from about a foot away. I didn't know whether to be creeped out or invite him back to my place (as it transpired, neither happened).
The second one was a few months later. I'd stopped to pick up some groceries, and on my way out of the supermarket, this guy who had been on his way in did a 180 and followed me out the door and to the corner. After crossing the street, I had to stop at a convenience store that was about a block north of the intersection. He was going east, and he stood at the corner and clearly wanted me to follow him.
What got to me about both of these was this -- why, if they were interested, didn't they just say "hello"? So I figure they weren't so much interested, as being creepy. (There's that low self-esteem thing again, I guess.)
?