First, people often spread propaganda without repeating it. There was interesting research a little while ago showing how propaganda stories spread across the Internet; most of the propagation is done unknowingly. Also, others simply use the techniques without realizing it. If they don't intend it to be propaganda, is it? I think it doesn't matter, unless we want to shift focus from the comment to the speaker - a falsehood is just as false independently of the speaker, no matter what the speaker's intent. Ah, uncertainty, a primary tool of the trade.
How do you point out propaganda, politely? One defensive tactic, both of the propagandist and of a normal person (again, it's never certain), is to frame it as a personal attack. Also, it's one thing to point it out with the personal distance of a online message board; what do you say to someone you are talking to - 'those are propaganda talking points'? That's rude, but otherwise you empower it. Again, it's a parasite on good faith and politeness.
This discussion is a tangent, but a very important one: What to do? Doing nothing, politely ignoring it, is to empower it. We've seen disastrous consequences from that, both historically and we seem to be heading in that direction now.