I don't see how it's relevant that everybody thinks they're being rational. That's probably true. Most people don't admit to themselves, much less to anyone else, that they're stupid or irrational. A lot of them would fail simple logic tests, but they still might believe that they're logical thinkers. They're just wrong in their perception of themselves. The fact remains that some are right and some are wrong in their perception. I don't buy this relativist idea that nobody is right, or logical or whatever.
Goodness, the law school entrance exam in the U.S. is one huge, multi-hour, horror of a logic test. They make up jurisdictions, laws, fact patterns, and you have to draw logical conclusions. Some people are good at it, and some aren't.
As an observer, you can judge by all sorts of things. Does the person proposing the argument ignore contrary evidence? Do they provide as proof things that actually aren't true at all? Are they overstating or misinterpreting the evidence that supposedly supports their point of view? Do they, in every situation, find that the evidence totally supports their point of view or favored group. Of course, then there are people who impeach themselves by showing their dishonesty. You can discount any argument whatsoever that people like that raise.
Aren't we on this Board supposed to be trying to present the latest evidence, from the best sources, in the most honest way possible so that we can come to the best, even if tentative, conclusions? I know which posters I think do that and therefore whose posts I most admire.
To look at it from a political point of view, I can turn on a left wing channel and a right wing channel to get the "slant" or talking points that each side uses. There are, however, certain politicians, certain journalists, who do try to present the issue "in the round", looking at all the complexities, examining all sides of an issue honestly. They don't usually get very far, but they exist.