Here is something I learned:
If something comes from an authority, from internal reasoning or carries emotional weight it is not the same thing as being true. Neither is the popularity of an idea or way of believing and thinking.
Also: if you fear some form of ostracism or punishment for expressing or even having doubts, that's another sign. There are so many!
To get around this I tried raising my standard of evidence (would an independent and impartial observer or test find the same thing?) and learning how to stand outside of myself and putting it all in perspective. Not easy, and I had to swallow some bitter pills as a consequence, but it's entirely worth it!
I had a couple of watershed experiences when I was about 7. As a result, I told myself that I that I should always be willing to be wrong, even if I had invested years and resources into something I believed was right.
At that age I first learned about the Nazis and was stunned. The other experience was having brainwashing explained to me. When I heard about these, I remember staying awake at night thinking that if I were careless, it might even be possible to brainwash myself!
All this leads me to a film recommendation. The film that sparked the brainwashing revelation was a Charles Bronson and Donald Pleasence flick called Telefon (1977).
It's about sleeper spies who live in suburban North America. They have no idea that they are in reality Soviet spies programmed to destroy The USA's infrastructure. They were activated by these words by Frost:
The woods are lovely, dark and deep
But I have promises to keep,
And miles to go before I sleep.
Miles to go before I sleep.
It shook me that someone could live a lie so thoroughly and not know it. It's funny, the things that shape us.