We're always having to decide when good is good enough:
Is my job good enough? That depends not only on your degree of unhappiness but on your viability in the job market and on the amount of time and energy you have to look for a job while still employed.
Should I change careers? Examine some alternative careers--by reading Googled articles, visiting people in that career at work, and perhaps examining what the training is like. For example, on colleges' websites, find course syllabi, the required books, and skim them the "Look Inside This Book" feature on Amazon. Are you likely to be more successful and happier in that career?
Should I marry this person? How would your prospective spouse rate on these items: sexual compatibility, non-sexual compatibility, stable employment, stable personality, kindness, lack of a serious weakness like a temper or addiction, thinks the world of you. For details on this Relationship Report Card, click HERE.
Should I rent this apartment? In the wake of home prices skyrocketing in major coastal cities (heavily because of massive buying by the Chinese,) rents have followed suit. So it's particularly important to take the time to try to find a below-market-rent apartment or house. Ask your friends for leads and in your desired locations, hang "rental wanted" flyers with a friendly looking picture of yourself and tear-off phone numbers at the bottom. Surprisingly often, the owner of nice homes have an attic or basement apartment or backyard cottage they'd rent below-market to someone they could trust. It's worth the time to try to unearth such a bargain.
Should I buy this car? For many people, the smartest buy is a few-year-old Toyota, Honda, or Subaru. These cars, reasonably affordable when new, are significantly depreciated after a few years yet have most of their useful life left. And those brands tend to be reliable for 200,000+ miles. My 2004 Toyota Prius has 276,000 miles, has had no major problems, and still runs perfectly. That has been the story with all six of the Toyotas my wife, daughter, and I have owned over our lifetimes.
Should I accept a diagnosis as accurate and the treatment as wise? Alas, medicine is still in its adolescence. So a serious diagnosis or significant treatment almost certainly is worth a second opinion. To ensure the second opinion is independent, get the referral from someone other than the first doctor. For example, if the diagnosis is a heart condition, you might call the cardiac care unit at a respected local hospital (Except for rare conditions, that's a good community hospital, not a research/teaching hospital) and ask the charge nurse for a referral.
Certitude is unrealistic. We all wish important decisions could be made with 100% certainty. Alas, that's rarely possible. So the wise person takes actions such as those above to improve one's odds but when additional investigation or rumination is unlikely to improve accuracy much, it's time to act.
Marty Nemko's bio is in Wikipedia. His new book, his 8th, is The Best of Marty Nemko.