Not exactly true. Nukes are more like submarines: once you've heard one, you can recognize it again. If there's a new design, it can take a while to develop an understanding of what it looks like.
On the other hand, the isotopes in the atmosphere is something that the folks in Sweeden would see pretty quickly, and there's no keeping their mouths shut (especially after Chernobyl).
> If NK set off a nuke they would know it and we'd be
> in full,pants-wetting, Defcon 1 mode.
I'm not sure I agree with this; PRK detonating a nuclear device is not a tactical threat: we might not even see a change to DEFCON 4 in response to that. It's a strategic threat, to be sure, but DEFCONs are meant to be used to define alert postures: ready for attack.
I don't think if PRK tested a weapon that we'd be bracing for an attack.
And finally: even if "they knew it" it's not clear at all that "we" would find out about it in a timely manner. DEFCON changes are not on the crawl: several of the DEFCON changes in history have never been announced.
/jordan