If the Norks somehow manage to deliver and detonate a nuclear device, how will anyone know it was Nork home-brew hardware and not something covertly imported from China or Pakistan?
Without going into the details ... we would know. Not all fissionable material is identical.
Each nuke has a definitive footprint. It has to do with how the materials are enriched. Within an hour, most any NEST team in the world will be able to tell where the materials originated.
A little “horse already out of the barn”, but that’s how it’s done.