It's much more than that. First of all, his "guesses" are based on extensive experience and a very deep knowledge of anatomy, pharmacology, etc. Second, whenever he comes up with a hypothesis, he always orders tests to confirm whether he was right (but sometimes, the only available test is to try something on the patient and see what happens). And he's a big fan of Occam's Razor (there's even an episode by that name in the first season, I think), always looking for the simplest hypothesis that will explain the largest number of symptoms. All of these things apply to debugging too.