Bonds failed the eye test. There was no doubt he was juicing.
The other big tell for both him and McGwire was the fact that they were hitting for power more as they got older, and they were still playing a lot of games. For most players, their power starts to drop off as they get older, and they have to learn to hit smarter. They also often get more days off. The steroids help them keep their power up, and give them the great combination of a veteran's eye and patience along with that power. Both of them were hitting harder in their mid- to late-30s, and still playing 150 games, when the mortals like Griffey were reaching the wall less often and only appearing in 130 games or less.
Sure, genetics and luck play a role in longevity too, but between their physical appearance and performance, some of the MLB standouts of the late 90s & early 2000s made it pretty obvious.
