I suspect that GSP, like just about every professional athlete, used PED's of one kind or another.
But if anyone has a system that can tell by looking at a picture then they're a rich man, because the IOC and many other sporting organizations will pay them millions of dollars for their visual system. Currently blood tests cost tens of millions every year, and are extremely inconvenient. Everyone would love to replace them with a visual test. But so far its only people on Sherdog and in gym lockers that know the secret visual tests system which is so much cheaper and better than visual tests.
Bro science says there are certain symptoms that only come from steroid uses. Medical science says for every symptom there are many (typically dozens) of non-steroid causes. Google "extended belly" for instance, if you think having an extended belly is a sure sign of steroid use -- you'll find sites like WebMD and the Mayo clinic which give you a long list of causes.
And then ask yourself, if you come down with a life threatening disease or get into a serious car accident, are you going to want the ambulance to take you to a hospital using medical science, or to the nearest gym to have a bro-science expert operate on you.
Did GSP use steroids? I'd say almost certainly yes, everyone at that level does. Can you tell by looking at him? No, and anyone who tells you they can is selling you something, because if they really could they'd be selling their services to the IOC and sporting bodies for millions of dollars a year.