It's not at all difficult to prove facts. Anyone who can prove that what is being presented as fact by news outlets are not facts, but lies or distortions, can show whether or not the news outlets are presenting propaganda. If it can't be proven as a fact, it shouldn't be presented as "news", or at the very least, it should be very clearly labeled as "opinion".

So for instance, if FOX was stating as fact the idea that Joe the plumber would have more of his money taken from him in taxes if Obama's tax plan was implemented, it would be very easy to show that this was not news but propaganda.