The problem is that most gym goers believe that if the next guy has one ounce more muscle, or is a little leaner, than they are on gear. I remember when I first started lifting weights in High School, the other kids accused me of roiding LOL. I went from 145 to 155, and this was a telltale sign to them.
Some people have great genetics, bust their ass, and their nutrition is dialed. Other guys are just straight roids. I had guys in my fraternity that were juicing, and had great gains on shit diets and crappy workout routines. Whenever they got off cycle, they dropped almost all of their muscle.
That being said, there is a certain type of facial bloat that only comes from roids, as well as pimples, an unnatural amount of sweat, yellow eyes and in women, a lowering of the voice. Also, you know when they talk about it. Like when you walk in the locker room and they are buying gear. Or they tell you their cycle.
I don't care, but be smart about it. Some guys are taking gear just because they hear it works, and there is no science behind it. I know a guy who took the same steroid cycle three times in a row. His receptors were gone after the first cycle. And some guys take different stacks, where one steroid counteracts the other. That's a waste of money, and not worth the additional strain on the body. Also, since it's illegal, you never can tell the purity of the source.