You need to first notice how propaganda works on you yourself. If you're not aware of that, then you're probably not trying to combat propaganda at all, you're just trying to replace the other side's propaganda with your side's propaganda.

Once you do notice how propaganda works on you yourself, you can start to track little movements of deviation from it. But this looks and feels a lot more like questioning your own beliefs than it does like trying to persuade others of the truth.