There are a couple of possible clues, but at this point that’s all they are. North Korean state media bragged that Pyongyang’s nuclear deterrence is now “diversified,” which might hint that it started using uranium. On the other hand, state media also claimed that North Korea had “miniaturized” its warhead, which would make it more suitable for use in a bomb or even a missile; Reuters reported that plutonium is the better-suited material for this purpose. Although when I asked Jeffrey Lewis, a nuclear proliferation expert who specializes in East Asia, whether or not this was accurate, he responded, “No, that seems to be speculation. Good news is the author has a 50/50 chance of being right.”

So how do we find out for sure? That “requires quick detection and analysis of the different types of xenon gases produced in an atomic explosion,” according to a recent New York Times story. The United States has special, highly sensitive airborne monitoring equipment and radiological stations around the world. The xenon gas has to be measured within the first 10 to 20 hours after the explosion, but the problem is that it can take a few days for the gas to leak out of North Korea’s underground testing facility.