I think it's a bit of both. Assuming a good pdoc, and not one of the bad ones.

All they have to go on is the patient's testimony, more often than not, our subjective reporting, to the best of our ability to even articulate some things. So I think in the beginning, all a good pdoc can do is try to make their best guess and try something. However over time, I would expect a pdoc to refine their approach strategically as they learn more and more about the patient.