Some things that make me trust and believe a documentary are
1)credible sources
2) coming from someone I can trust
3) if passes the eye test, if it looks real.
the thing with all of these is that it is never 100% certain. for example a “fact” today can be debunked tomorrow.
someone you trust can forget their morals every now and again. it doesn’t even have to a person, it can even be a company. (rip history channel)
and lastly with the amazing cgi that exist today it would take a trained eye to detect all of the fake viral videos that exist today.
I know what you mean. To add to what makes a credible documentary, a director can also use different techniques to make his film more believable. Or the filmmaker can use your limited knowledge on the subject to make you believe anything he tells you. A good example of this is “Nanook of the North”, where the director used the audiences lack of knowledge on Eskimos, in order sell them a fake story as fact.