Going by stats alone can be fairly difficult because of how much variability you can get from one sample to the next. Skinny guys in the pre-steroid era demonstrated big swings in power numbers for one season. Sometimes, guys are just lucky or dealing with chronic injuries that push numbers in one direction of the other. To me, the clearest evidence comes when changes in muscle mass are extremely unlikely from natural causes. To go from fairly thin to huge and ripped long after your natural testosterone levels have peaked is really fishy. I know guys who are that huge and ripped without steroids, but they looked like that in high school because their genes made it possible. Unless there's some crazy epigenetic stuff going on that we don't understand, you don't develop a genetic endowment that makes your veins pop out all over when you're 30. Of course, not all steroid treatments will make you huge, but gigantic gains in muscle just don't happen without "help".