July is a wonderfully conflicted month to be a fan of endurance sport. It provides the excitement of the Tour de France, and the international track and field circuit is in full swing. But with both come plenty of opportunities to wonder, justifiably, whether you're watching something clean or tainted.Just this week, the Tour de France flew through the Pyrenees at speeds comparable to most performances seen in days when doping with oxygen vectors like EPO was common. Yet Lance Armstrong, Marco Pantani, Michael Rasmussen, Jan Ullrich - all confessed dopers from that era - would have been generally equally matched against the current group of top riders.In athletics, a world record that could well have been drug-assisted was broken when Genzebe Dibaba of Ethiopia ran 1500m in 3:50.07. The old record was one of "those" Chinese records from the 1990s, a period when Chinese women athletes broke the 1500m, 3000m, 5000m and 10000m records at a pace never seen again.But comparisons with history, and proximity to cheats, either in time or geography, is not by itself grounds for doubt.Physiology and performance are too complex to afford the luxury of absolutes. If you watch too much CSI, you might think of "proof" in absolute terms, DNA-type evidence that is incontrovertible and definitive. Physiology does not allow this, and neither does performance. No numbers, power or stopwatch will prove or disprove credibility.All we have is probability, allied to a systematic (or scientific) approach to those numbers, to get a better understanding that we might add to our other "senses" to understand what we are seeing.There is simply too much "noise" to become a cynic based on numbers alone. I would call this common sense, but so far, some of the responses to this approach in the Tour de France suggest it is not nearly common enough.Imagine for a second you are an alien, visiting earth to watch your first Tour de France. You watch the cycling, and perhaps wonder why two teams (Sky and Movistar) dominate the mountains so comprehensively? You could easily accept this as "normal" because you have no frame of reference.Unfortunately, or fortunately, we do. That frame of reference includes history, context and knowledge. And if someone who DID know, and was qualified, explained to you, the alien, the history of cycling, and how every champion from the 1990s onwards is tainted by drugs, and that the balance of risk vs reward remains skewed in favour of risk because of new undetectable drugs, clever doctors and micro-dosing, then you might view what you see a little differently.If your earth host explained that cycling used to be dominated by a team that did pretty much the same thing you have seen from this Tour, and this team was exposed as one of sport's greatest cover-ups, then your interpretation of that dominance should shift, not to the point of judgment, but certainly circumspection.And if you were told the performances are comparable to what they were with doping, then you might cross over into the realm of scepticism. The same is true for Dibaba, and Usain Bolt, for example, whose 9.58s made him so much faster than so many known dopers. Is it possible? Of course.And we shouldn't cross over to outright cynicism - medals are won by 100% clean athletes, so the generalisation "they all dope" is as false as the one that dismisses any scepticism as if it were a verdict and judgment.Somewhere, there is balance. The "fanatic" root of the word "fan" makes that difficult to find...