Sounds like, dispite claims to the contrary, it isn't all that difficult to detect whether uranium is weapons grade or not.
There are a few complications, to be sure. Each decay type sets off a series of subsequent decays, which contribute some additional radioactivity from much shorter half-life isotopes present in the sample in small amounts. U-238 reaches a stable form of lead only after 8 alpha and 6 beta decays, for instance. Which means for higher precision you need to include correction factors for a zoo of additional isotopes. But the basic idea is not difficult to understand. The "head" of each decay series is the "rate determining step" for the subsequent quicker ones, because they are so much longer half-lives.
It is also important to understand that U-238, while radioactive (meaning, it emits alpha particles etc, and thus some energy and heating) does not split ("fission") into two heavy nuclei and release large amounts of energy. It is "breaking down" to be sure, but only splitting off tiny helium nuclei (alpha particles). U-235 can split into heavy nuclei, if hit by neutrons moving the right speed, which is the process (if "fed" right to "feedback" onto itself, to set off a chain reaction) that drives atomic explosions. The ordinary radioactive decay of uranium into lead is *not* the process involved in nuclear explosions, just speeded up or something. No amount of U-238 explodes.