One finds the truth the same way it has always been found. You have to look at statements of fact only, not opinions. Statement of facts are things that are possible to support or refute based on known data. Everything else is opinion or speculation, and should be regarded that way.

The hard part is when you hear such an assertion, you actually have to fact check it yourself and not rely on others to do so. Until you've done that, the assertion of the fact checker should be considered only possibly true. All of that takes work, of course.

If you're going to (as you often must) accept the word of others for something, then keep track of it and compare what they said with what ends up being true in the long run. You'll pretty quickly learn who is more likely than not to be right.