He employs a ex-pro bodybuilder to test his drugs for him. He makes sure they are 100% pure before he injects them.
Canelo hasn't actually knocked anyone out since moving up other than Kovalev and it looked mostly due to exhaustion
Canelo uses and has tested positive for Clenbuterol in the past. Clenbuterol has a half life of thirty six to forty eight hours so after two or three days off it, tests show nothing or trace amounts. Therefore any testing could be passed by proper cycling.