I've been at my gym so long that I can usually tell someone who is on "something." That's usually because I see them literally grow in the space of weeks or a few months. It's the size coming on quickly that gives it away. Sad thing is I also see them lose it just as quickly.
I'm pretty sure though that everyone has me branded though since I guess I've consistently kept growing albeit slowly. There are only a small handful of guys at my gym that are really big guys larger than me, probably less than 10. I'm often the biggest guy at the gym at the time I'm working out (and I'm not even that big) so I'm pretty sure everyone thinks I'm a user. If they ask, I usually tell them yes, I'm on, but because I have to be.