This ought to be highlighted, before people start an echo chamber of "doctor's don't know what they're doing!" that ultimately undermines everyone (guess what argument the anti-vaxxers use).
A string of anecdotes of "I double-checked and it turned out the doctor was wrong" remains less than a trickle compared to the flood of patients doctors have to deal with, and a tiny fraction of the hypochondriacs they must deal with. Our medical system is broken enough without adding doubts on the competence of its medical professionals.
By all means, people should double-check what the doctors say, but realize the much worse consequences of sowing doubt.
Another useful one is to ask him to explain his reasoning.
If he can't justify his decisions, then he's probably using instinct which is unreliable in the case of rare diseases, or he doesn't know how to diagnose known diseases which means he's incompetent.
Some people are offended when you challenge them for reasons for their advice, but I think that's because they aren't confident in it themselves or feel superior. Doctors shouldn't be in either of those positions and should be willing to tell you how they came to their conclusions.
Whilst on the whole I agree with you, the amount of doctors I know that would react (quite strongly) negatively to that is quite high, so anyone who takes your advice should be prepared. The most common response I've seen to your question was the doctor reminding the patient "there is always something called a second opinion", a couple of times though I noticed them taking the time to explain it.
Doctors and patients do not know what you mean when you say "your risk has gone up 50%". When you say "in a group of ten thousand people we would expect 4 to experience this thing. But in a group of 10,000 people who eat peas we expect about 6 people to experience this thing".
Gerd Gigerenzer has a book explainin it better than I do.
This highlights the importance of a second and third opinion as well.
As the joke goes, "what do you call the guy that graduated last in his class at medical school? Doctor."
Doctor's aren't machines, they're more like mechanics, they see a symptom, use their knowledge to make an educated guess essentially, and then work from there. If their knowledge is soft it's likely an incorrect diagnoses could just be exacerbated by incorrect treatment.
Get another opinion, and then another, the next problem is deciding on siding with consensus or outlier.
A string of anecdotes of "I double-checked and it turned out the doctor was wrong" remains less than a trickle compared to the flood of patients doctors have to deal with, and a tiny fraction of the hypochondriacs they must deal with. Our medical system is broken enough without adding doubts on the competence of its medical professionals.
By all means, people should double-check what the doctors say, but realize the much worse consequences of sowing doubt.