Having bad days is not a proof of being clean. Jan Ullrich often had bad days in the Tours when he got smashed by Armstrong, that didn't mean he was clean.
Basso shipped 40 minutes on a Giro stage in 2005, he wasn't clean.
Landis lost 10 minutes the day before his insane recovery to Morzine. He wasn't clean.
In general the easy things people like to point to, things like "rider X is always at high level", "rider X is always in insane peak at his season targets", "rider X is not stable" or "rider X doesn't look tired" are all basically meaningless when it comes to answering whether a rider is clean or doped.