The simplest means by which to differentiate propaganda and dialogue is to ask this question: are you discussing ideas? This is dialogue.
Propaganda is the process by which all purported interactions take the form of discussing people, or the discussion itself. For example, reducing debate to labelling (You are a right-winger, therefore I have nothing left to say to you). This is absolutely the rejection of debate and dialogue. It is proclaiming a unbridgeable chasm between that person and someone else thereby identified as an ideological “Other”, and hence eligible for hatred and derision, regardless of the content of their speech.
Alternatively, you often see the discussion of the discussion. Can you make general claims about black people? Mexicans? Leftists? Can you say “leftists usually are. . .”?
Self evidently, in their concern about “essentializing the Other”, Leftists will fall all over themselves to reject such discussion. Except when the topic is right wingers or Christians. Then it’s all good.
These sorts of blatant, repeated, Grand Canyon-esque (all intellectuals need to work -esque into things; I’m told it makes you look smart) gaps of logic characterize propaganda, which is really nothing but the substitution of mutable emotion for mutable reason.
Go to any message board and watch the discussion. You will see conservative propaganda, too, but in general most of the name-calling and “discussion of the discussion” (DoD) will come from one side of the aisle, and one side only.