glance, don't stare. at the gym, you really gotta wait for the right moment to check someone out. your primary focus should be your workout. don't go outta your way to get a glimpse of dat ass, you know?
don't hit on a girl during her workout unless you're already friends and you spend less than a couple minutes doing it
also, people saying never hit on a girl at the gym either don't go to the gym, don't have any game, or generally don't know what the hell they're talking about (see above).
source: i work at a gym. people flirt with one another all the time. sometimes it's too much, sometimes it hits the sweet spot.