You know what should interest you, is that YOUR country is astroturfing (and probably much more than NK). There's probably 99% chances that you will detect NK astroturfing (they're not natives), but how well can you detect astroturfing from your country ?

~Easily. If they espouse opinions contrary to my own, they must be government shills.~

~Oh, hang on. I accidentally left my foil hat on. Let me take that off.~

It only takes a slightly elevated level of awareness. Usually, just knowing that it is possible that a participant in the discussion could be a paid propagandist is enough to spot likely candidates. While you can never really know for sure, if you check their posting history, and they only seem to care about exactly one thing, that's a strong indicator of suspicion.

On some of the message boards I frequented in the past, the general subject matter was such that it was extraordinarily easy to spot the cop whenever one showed up. They always did exactly the same thing, and it never worked even once. It usually went something like this.

  A: Hey guys, we should get together and *do* something.
  B: Sure.  Tell us your plan and we'll tell you if we're in.
  A: [*describes an overtly criminal act, usually involving violence*]
  C: Go back to writing traffic tickets, undercover cop.
  D: Do you actually get paid to annoy us, or are you doing it on a volunteer basis?
  B: You should take a look at your bosses instead of spying on us.
  E: [*spouts unrelated crackpot claptrap*]
  (end of thread)

On a more general forum, such as HN, with a much larger and more diverse audience, it is a bit more difficult to spot the paid propagandist. And the professionals have honed their skills in the past few years. They spend more time setting up for the payoff, building reputation and credibility, before using an identity for a mission-critical message. They almost certainly have voting rings set up on every site that uses voting.

Anyway, any genuine NK astroturfers would probably be doing it from a non-NK IP block. They're just brainwashed by their cult-like government, not stupid. They ones trusted to access the Internet to further the interests of their leaders almost certainly have access to Chinese, Russian, and American IP addresses.

Spot the shill might be an interesting game. How would you go about writing software to do this?

Might be harder as operations are aware that people can sniff out a shill or sock puppet if they have five minutes and cared.

Tempted to give this a shot anyway.

Russia do this too.

I imagine they're smart enough to use VPNs or something.