I ask this because the Kersee thread has popped back up. He was well-known to be doping his athletes in the '80s, and in more recent years has coached many of the world's top sprinters (Felix, McLaughlin, Mu).
Even on that thread, specifically about Kersee's history, most of the back-and-forth on whether Felix and McLaughlin are likely to be dirty is based on their progression/appearance rather than on the concrete fact that for many years they have run for a notorious doping coach.
As a comparison, suppose that Lance Armstrong decided to coach cyclists. He starts getting good results, and soon he has some of the best young athletes flocking to his team, where they continue to improve and become world-beaters. Would we say things like "this athlete was a star as a teenager, no surprise he kept improving" or "he looks nothing like Barry Bonds; he can't be taking drugs!"? Or would we say "this athlete deliberately sought out a coach who is best known for doping his way to success, a coach who would certainly choose to dope athletes if he can get away with it. Whatever the specifics, the athlete is almost certainly dirty".
It's an impossible question to detail exactly how a given athlete's doping program works. Did they start doping in high school, gradually over the years, or later in their "natural progression" when their improvement would have otherwise plateaued? Could be any of them! Is their appearance relatively normal because they're microdosing, because they're taking EPO instead of steroids, or because they naturally have a baby-face that drug use only hardens so much? Many things are possible!
An athlete can't necessarily choose their own progression. They can't necessarily chose their own appearance. But they can ****ing well choose their own coach. Any athlete who willingly decides to work with a known doping coach over a period of years should be assumed guilty, much more than anything else short of a positive test.