Well, at my gym there are three guys that come to mind. Even though it is more a tangent than an answer to your question... Lets do their ranking:
One is a 60ish y/o guy that I almost never see on the gym floor but have seen at least 10x in the locker room flitting back and forth from one end to the other. I've seen him check me out. (annoying)
One looks to be about 25 and flabby as if he's lost a lot of weight. I have seen him on the gym floor occasionally, but I usually see him flitting in and out of the locker room, and I swear he takes multiple showers. I think he checks anything out with a pulse. (creepy)
The other guy I think buddy listed me on this website and I usually see him on the gym floor, in the sauna, and the lockers equally often. (if it is the guy) (normal)
I don't think you can always be sure whether they want you or want you to stay away. You can however, tell the creeps from the regular guys quite easily.