"Whenever somebody with questionable intentions or knowledge is presented with this question, they will usually struggle to answer (watch them squirm, it is entertaining…). It is a lot easier to tell if they are trying to pull a quick on on you by listening to their answer to this question, than by listening to their (perhaps rehearsed) initial statement or opinion.
If you are not convinced, ask again "How do you know that?" until you get a satisfactory answer.
If all you get in reply is: "I read it in a magazine" or "I saw it on my friend's facebook status update"then continue checking and doing your own research
If you get "I'm a health professional. It was published in a non commercially sponsored, peer reviewed study, and it works for a large proportion of my large client base who's progress has been tracked for months" then its a lot more likely to be correct information and advice for you.
Try it on shady salespeople, Internet forum "gurus" who are just giving you their (usually uninformed) opinion or even a friend who may a great friend but may not be in the best position to help you get results.
Especially in the age of Internet where opinions abound but facts are hard to find, I like the point made by author Isaac Asimov. "Democracy means that my ignorance is just as good as your knowledge"."