Because IAEA coordinates and monitors NPT-treaty there is lots of open source information about the subject. Seismographic analysis has become really accurate.

Because IAEA coordinates and monitors NPT-treaty there is lots of open source information about the subject. Seismographic analysis has become really accurate. 0.1 t .. 10 t aka (extremely-low-yield testing) can be concealed from seismographs. Very-low-yield testing between 10 t and 1-2 kt is very hard to conceal, and everything from low-yield testing up (>20kt) can be detected with certainty.