My understanding of nukes is there is a signature to each one based on the materials used. As a consequence, you can trace it back to the manufacturers (read: countries). While I doubt NK would be good enough to submit a sample, by process of elimination, it would be figured out. And that is where the fun begins. How do you stop the next bomb from being detonated? Is it already out there? If yes, where? If no, how do you stop the production and dissemination of the bombs.
It's all theoretical until some town/city somewhere gets vaporized. That will supply the motivation. The question is, the motivation to do what?
Very hard question.