HAHAH
What are you talking man?
Claiming some of these athletes for example in Olympics are clean is same as claiming that somebody could step up on Mr. O -stage clean.
The testing done in Track and field on an INTERNATIONAL level is pretty stringent.
So I think it's safe to assume that when they compete in the Grand Prix's, they're clean
It really boils down to if the respective national track and field/Olympic organisation is actively testing their athletes when they're at home in the off season.
Some athletes are also famous/notorious for ditching the Grand Prix's, staying away training for only the big championships.
Eg, the Greek sprinters Thanou and Kenteris who both got busted in the end.
IMO, at least USA, Germany, UK, a few of the Scandinavian countries, et al, have fairly solid testing.
Some other countries don't have the same programs.
Eg, in Weightlifting, the recent phenom Pat Mendes is going to compete for Brazil instead of USA.