They don't bluff. They talk smack and then attack a target of opportunity, like a spy plane or a destroyer. In this case, they know that a miscalculation can throw them into a war they can't win, so they haven't mobilized their army. I expect a provocation from their navy or Air Force as that's their history.
North Korea has a history of "Sabre rattling"... which means making a lot of noise, but not really doing much of anything else. With any luck, Kim Jong Un, is just trying to live up to the military hype of his father and not cause real damage. I wouldn't worry too much.
Well, it's a lil complicated, but I have a theory... Ok so, North Korea is bluffing bit they are only saying that because they think that we will attack them? if we attack them then another world war will break out because we are a large "wealthy" country and we could easily take out North Korea, they know they can't defeat us alone? so they think that we will attack them and the world will help them take us down? so in conclusion yes, North Korea is "bluffing"