Let me phrase my objections another way:
At what point do you, as an outside observer, become satisfied that someone else HAS done sufficient research, and her decisions are valid and should not be criticized? What evidence would you have to see to make you feel that a woman who had a c-section or an induction had considered the decision carefully enough to be considered a thoughtful consumer?
I don't see those as objections but fair questions.
There's no one measurable place to draw the line, but it would certainly have to go beyond what the woman said in the OP:
Well, clearly that woman should NOT have been trusting her doctor because (assuming the story is accurate, which is admittedly a "maybe" since none of us were actually at that doctor's appointment),recommended something that goes against the evidence and poses unnecessary risk--namely, an induction based solely on Naegle's Law with no medical indication. Whom we choose to trust is based on our personal feelings and not always what is ultimately best for us. The most ethically minded physician with the world's greatest bedside manner could still go against the evidence.
To answer your question, I think I would have been satisfied with anything other than the aforementioned quote.
"What if I did wait until I went into labor naturally? What would be the risks and benefits?"
"Do you see something medically wrong with me or my baby that would require this?"
"I've been your patient for many years, and I really respect your opinion. So I hope you don't take it personally if I seek a second opinion and get back to you with my decision."
There's no Magic Bullet question to ask, and there's no universal measure for savvy consumerism and healthy skepticism. But it certainly goes beyond letting other people, who may or may not share your interests, make monumental decisions for you. You'll never hear me or any other critical thinker say, "I trust my car dealer to make that decision for me."
Questioning somebody's advice or decisions doesn't make you fundamentally distrustful or cynical. It doesn't mean you dislike your doctor or think all doctors are evil. If Mother Theresa herself were my car dealer, I would still ask to see the Car Fax.