It always makes me laugh when people hold on to the belief that juicing is somehow rare, that there's no way that all of those big-skulled, thick-bellied, huge-muscled dudes could *possibly* be doing them.
Sure, some people who look like they're doing them aren't. But they're a lot fewer than people who look like they're doing them and ARE.
And there are some people who don't look like it, but are. Even more. The crazy thing is that roids aren't just big in the pros any more, it's insane how many high-school kids are doing them, too. One of my cousins' family has always been HUGE into high-school football, all of their kids play, are champions, yadda-yadda. Sometimes half of the team is juicing up, it's not exactly a secret.
I haven't been able to find corroborating evidence, but I've seen claims that CSAC has a 90% positive rate on their tests, giving a full 25% of their fighters a positive at one time or another... and of course, more people are doing them than are caught.
I think that if you stood all of the UFC fighters in a group and threw a golf ball into the crowd, chances are 90% you'd hit someone who *has* used, and 25%-50% that you'd hit someone *currently* using.