Overall, it's usually um, the attractive ones? Haha, obviously. If I dig a guy's style, that'll usually catch my eye at first (although it doesn't matter much to me in the long run). And if they give off a good vibe and seem like a chill person, if they seem interesting. Good hair and/or facial hair will also get my attention, LOL.
The only guy I've ever "checked out" (LOL, I dislike this term for some reason, but I digress) at the gym was this scruffy indie-type guy eating soup and reading in the cafe at the gym, hahaha. I usually don't find too many of the people there especially attractive.
Guys I don't really look at are super clean-cut (like, wearing all Abercrombie or something, I dunno), Jersey Shore-type guys, frat-type guys, guys who seem really arrogant and/or stupid, haha.
I like guys with nice hair and friendly smiles that look approachable. Now the types I don't even bother to look at are, fat guys, anyone that looks like they're from the jersey shore, guys that dress preppy, guys who wear more make up than me, guys who are stupid, guys who are younger than me, jocks, guys who look wimpy, and guys who look like Justin Beiber. But my ideal guy doesn't have to be tall or muscular, as long as they are taller than me (5'2) and as long as they are not fat I'm happy.
I generally look at guys with a decent face, nice hair, broad shoulders, masculine and who are fairly tall. I don't look at guys with ugly faces, feminine, fat, bad hair, skinny, small shoulders. This all only on face value though.