Well, 3 MVPs BEFORE hetting involved with BALCO should be enough for most people to not doubt the man's skills.
He always had a lot of skill but you dont go from guy who for ten years had trouble hitting 20 homers a year to what he became. Plain and simple, he used chemicals to alter his body.
Other than the 73 homer year he's been consistant. I don't know how you came up with 20 HRs but other than the one year he hasn't been over 50. Consistancy.