I guess what I'm trying to say is this: how do you detect a nuclear device?

Geiger Counter?

Something like this?

No matter which material you have stored, it'll trigger both of these devices.

You'd need to do a comprehensive study of the material to determine it is fully enriched -- if indeed they are using uranium. Again, they could be using any element of nuclear materials.

Since you can't stop the nuclear material from decaying and sending off detectable radiation patterns, the best place to hide the stuff that goes boom is among the stuff that goes fizzle.