NOYB wrote:

We'd know in a heartbeat if a Russian nuke was detonated on US soil.

How?

From the isotopic signature.


Bull****!! Their radionuclide composition (and thus their age and
origin) can be determined. BUT, "origin" in this case means what base
materials it came from NOT what country!! If you don't have an example,
you'd still not know even after obtaining the isotopic signature.
Signature is used for things like, say Chernobyl. They have the
original material, the reactor fuel, so, when they find a radionuclide
compostition, they can then compare the signature with the original.