While there are many truths to this, there is at least one exception. At the high-end, when circumstances are physically challenging enough, people will betray effort in some ways that are visible. It could be during the effort or immediately following. This is even more prominent during a long road race compared to say, a time trial, primarily because for a really fit athletes, a time trial is not going to be limited by endurance, and going over capacity during the TT is often counterproductive.
But each to their own opinion. I have often found that for our favorite riders, we will put the blinders on at least partly if not fully. I have been guilty of this myself, not only watching races, but being in them. I have ridden with some really awesome and talented guys who were also rampant dopers, as well as complete pr*cks who were simply hugely talented but clean. But the dopers always made it look that much easier. At my current level and advanced age, if somebody rips up a mountain like I'm standing still and looks effortless, that's just life. But at the elite level, to make that kind of a gap and keep it going while riding within yourself, it makes you a huge outlier which in and of itself is fairly suspicious in the professional sports world.