As far as I can tell, the way most people decide this is: if I don't like it, it's propaganda
I don't think that "most" is accurate.
It is not hard to recognize propaganda, even when it coincides with one's established views.
In elementary school we had many quizzes and drills on this sort of stuff as part of our civics courses. The problem is, I doubt schools teach this stuff anymore, even though this kind of very basic critical thinking is very important for a functioning society.
I'm talking about the things people say on internet forums like HN about propaganda and related terms (astroturfing, shilling, brigading, foreign agents). At least on HN I can tell you that "most" is an understatement. That's plain from the comments and even more plain in the voting data. (I should make that more precise, though. Obviously I'm not reading people's minds to know why they say 'propaganda', etc.; it's rather that what people say about this is overwhelmingly correlated with whatever their view on the underlying issue happens to be.)
Elementary school is a pretty different context. Schoolchildren may be more reliable on this, actually. Although they tend to imbibe the views of their parents, they haven't yet had time to build up the same fixed emotional habits around them.
Those civic classes only prepared people for the 20th century world of propaganda comprised of overly-enthusiaatic pamphlets and loudspeakers, and to their credit did a good job of it. The problem is that those people are still unequipped to decide whether WeebFoxMaster13 is a real person or some marketer being paid $10 an hour in order to sell them on the idea of political activism
I don't think so. Propaganda works by convincing you without making you suspicious. If you can recognize it, it has already failed. Of course, not all propaganda is actually concerned with convincing you. Most of it is probably only concerned with convincing a majority, your minority opinion might not be worth the effort.
I don't think that "most" is accurate.
It is not hard to recognize propaganda, even when it coincides with one's established views.
In elementary school we had many quizzes and drills on this sort of stuff as part of our civics courses. The problem is, I doubt schools teach this stuff anymore, even though this kind of very basic critical thinking is very important for a functioning society.