GSP is extremely worried about his long term health, which would argue against him using something with long term consequences (most steroids taken at the levels and duration used by athletes). Fedor is religious, which would argue against him breaking one of the commandments by lying -- he'd admit it if he was using. Same for Khabib, religious so wouldn't lie. And you can go through almost every fighter and find philosophical reasons why they wouldn't take PED's.
People believe the fighters they like are the exception, and always find a rationalization for that belief. But then never believe the rationalizations for fighters they don't like. Almost all of people's opinions are biased (including mine of course) -- visual inspection doesn't work (its why not a single sport or testing organization on the planet accept visual signs as evidence), philosophical arguments don't work (people can rationalize anything), religious arguments do't work (people break their religious vows all the time). We're left with biased gut feelings about who is and isn't using.
My bias is that Nick, GSP, Fedor, Khabib, and everyone else at the UFC level is using. Some are using more than the test-safe amount, and test positive. Nick, GSP, Fedor and Khabib in my opinion are sticking to test-safe dosages and so pass tests. Other fighters took a risk went over the test-safe dosages to get an advantage over the fighters who limited themselves to test-safe, and got caught. In that sense Nick, GSP, Fedor and Khabib are among the better ones -- they never used more than the 'allowable' limit.
