When I see an "unbelievable" athletic performance, I never, ever like to engage in doping speculations because it's not fair to the athlete. It's also extremely likely that an athlete breaking a record or winning a medal is competing on a level playing field with his/her opponents. So if today's Tour de France winner is doping, you can easily conclude that 2nd, 3rd, 4th, and 5th are as well.
I'm making a bit of an exception to speculate about the astonishing new women's 1500m world record achieved by Ethiopian Genzebe Dibaba in mid-July at the Monaco Diamond League meet. She ran 1500 meters in 3:50.07 (converts to about a 4:07 mile). She broke the existing record by a few tenths that was held by a Chinese runner for 22 years. Dibaba just won the World Championships at 1500m also. After a slow early pace, she finished the final 800m of the race in 1:56. This is fast enough to win most every Olympics and World Championships at 800m!
The old 1500 meter record of Yunxia Qu record is widely regarded to be one of the most suspect performances in history, and possibly unbreakable due to the more pervasive doping of the past era and better testing of the modern era. Similarly, no one has come anywhere near FloJo's world records of 10.49 for 100 meters and 21.34 for 200 meters, which now stand for 27 years. This includes great runners and confirmed dopers like Marion Jones (10.65 and 21.62 personal bests.).
Yunxia Qu and her coach Ma Junren were widely regarded to be the most suspicious characters in the history of track and field. Ma and a small group of elite Chinese female appeared on the world scene from total obscurity and started obliterating world records left and right. You may recall that when Ma faced scrutiny about doping, he crowed about the women's intense training camps running a ridiculous 175 miles a week, and also how they drank turtle blood for strength. When six runners from Ma's camp failed doping tests before the Sydney Olympics, they vanished from the planet and we have seen extremely minimal representation from Chinese female runners in the elite ranks in the ensuing years.
If you look at the women's all-time 1500 meter performance list, it's pretty much a joke. The top 15 performances (ranging from 3:50 up to a more reasonable 3:55) are populated by the Chinese athletes from the mysterious Ma era, as well as Eastern bloc athletes during the era which has now been confirmed to be absolutely laden with doped athletes, especially female. The record Yunxia Qu broke had itself lasted 13 years, set by a Russian athlete Tatyana Kazankina, affectionately nicknamed "Ted" by cynics of the time.
Dibaba smokes the best women in the world, and then seems barely tired at the finish line? It's almost unbelievable to watch. She credited training with world class men and obtaining excellent coaching from former elite runner Jama Aden for her success. In contrast Shannon Rowbury, who took third in the race in a new American record of 3:56, has worked hard for 7 years to improve her best time from 4:00 in 2008, which marked her initial arrival on the world class scene. She recently took 6th at the World Championships.
The general sense from anti-doping experts and athletes alike is that we are steadily marching toward cleaner sports, more efficient testing methods, more justice for violators as well as those unfairly accused. Even MMA is trying to get cleaned up, with the recent hiring of Jeff Nowitsky to oversee anti-doping, he of BALCO and Lance Armstrong fame. However, with the stakes escalating higher and higher in elite sports, and the sophistication of pharmaceuticals and genetic engineering escalating as well, we will likely be facing these same problems and corresponding suspicious for many years to come.