How do We Know what We Claim to Know About North Korea?

I'm no apologist for the North Korean government.  That said...

Although it's certainly possible that 80 people were in fact publicly executed, JoongAng Ilbo is one of several Korean newspapers accused of heavy distortions.  And the following excerpt from the HuffPo article raises a huge red flag for me:

The conservative JoongAng Ilbo cited a single, unidentified source, but at least one North Korean defector group said it had heard rumours that lent credibility to the front-page report.

Not very convincing.  Neither of these could claim to be an impartial observer.

Further, if these people were executed in public, this must mean that the North Korean government wanted to send a message to its people (a message deterring them from watching smuggled South Korean TV shows) which, presumably, would have also been communicated through its media.  And while I'm certainly no expert on the North Korean press, one might expect that it - and not just a single unidentified source or a defector group - would have reported the executions (just like how the official media in Iran covers public executions there).  Perhaps it did and HuffPo simply didn't do enough digging.  Or, maybe my underlying assumptions are invalid.  Either way, who really knows what's going on?