“Who gains from people believing this?” is often quite the eye openner.
All the way back in the beginning of the talk about Global Warming if you did that analysis on information on the subject you would get the answer “a handful of scientists who actually risk being unpopular so not really gaining” for the “there is global warming” messages versus “companies worth trillions of dollars who want to protect hundreds of billions of dollars a year in profits” for the “there is no global warming” ones.
Nowadays we all know how that went.
Such answers to quo vadis are not a hard rule for a real/fake decision, rather they’re a signal for “needs more validation” since if there is one side which clearly stands to gain much with a certain message being widelly believed (or loose much if the opposite message is believed) and has the means to significantly influence the information landscape (normally that’s lots of money), the probability that the message is manipulated in a direction that favours them increases a lot.
(Here’s another example: if people who own lots of a specific financial asset - for example Bitcoin - tell you great things about it, you should probably be suspicious of that message).
Note that this alone doesn’t address the “somebody whose honesty is entirelly beyond reproach is a gullible fool and believes and repeats as if absolutelly true complete total bollocks” problem (which is how the worst misinformation spreads in places like Facebook - via genuinelly well-intentioned gullible friends and family). Ultimatelly the stuff that comes in via others needs to be traced back to the source “i.e. were have you heard this from?” or be treated as not trustworthy until proven otherwise even though it came via an honest person.