[exmadscientist]

There are some radioactive species that occur only as fission fragments -- no irradiation of naturally occurring isotopes will reasonably produce them -- and so from there you can get back to the original isotopic composition of the fissioning materials. There are enough tell-tale signs of each of the major isotopes that it is easier to separate them out than you might think. It is very, very possible, for example, to use a neutrino detector to tell exactly what is going on inside a nuclear reactor's core, which is not something you might realize is possible until you've seen it done! (But neutrino detectors are very large, must be placed very close in, and are very, ah, obvious things to have around.)

This analysis is quite difficult, and I don't know that it has ever been done on an actual detonation (at least, no one admits to doing it; not that there have been terribly many opportunities to practice this), but there is no reason it can't be done.

(You're probably right that Clancy took some authorial license, though.)

For an example of what is possible, see the ruthenium mess in Europe from a few years back: https://cen.acs.org/safety/industrial-safety/caused-plume-ra... or https://www.nature.com/articles/s41467-020-16316-3 among others.