One instinct I've learned from software engineering and the internet is that professionals are often wrong. The symptoms of a bad doctor are very similar to that of bad developers. Same with bad cooks, bad contractors, and so on.
The main symptom is contempt. You ask them a few questions or mention a second opinion, and their face turns into a scowl. Some are outright insulting, saying that X is a moron, without having enough information to make that judgement. Whenever I meet a doctor like this, I look for a second opinion, and very often it contradicts.
The next is buzzword. My child had a fever for a very long time and was hospitalized. A good pediatrician said "It could be Kawasaki disease... but it doesn't meet any of the other symptoms". A not very good one said "It's definitely Kawasaki disease, can't be anything else."
The third is well, passion. I know everyone hates the word, but I find a correlation. The worst doctors I've met are often trying to do anything except their job - they pitch unnecessary supplements, overcharge, talk about their stock investments. The great ones try to make you healthy; they end up undercharging, recommend cheap alternatives to their pills, volunteer for charity organizations, waive their consulting fees, and so on.
I realize this is a very subjective and biased way of looking at it, but it's worked well for me.