One of the unfortunate things about all the doping bit is that we, the observers, are left with few ways to determine if someone is - or isn't. One of the methods that has developed is the "eye test". Which, when considering it, may seem pretty weak.
But, this article actually gives physical validation to the "eye test":
http://sports.yahoo.com/news/mma--luke-rockhold-vitor-belfort-unnatural-physique-trt-use-231700682.html
Worth noting: the doper is openly doping, with TRT (testosterone replacement therapy). So he is openly jiggering his body's natural balance. The guy saying he "doesn't pass the eye test" is a competitor, therefore has some experience, and thus expertise in the subject.
Certainly not scientific, but when we don't have "scientific" methods available, expertise is the best we can hope for. Imo, this gives us some baseline validation for an "eye test", at least for steroid use. The photo that is with the article isn't the best - but the guy does obviously have a very low body fat percentage, which has been discussed in other threads. He's below the "normal" threshold.