All we can do is make progress. The CTBT Verification System . If the test is very small, the seismic waves may be too weak to detect at remote stations. The seismic waves arising from the tests would get lost in the noise of tiny earth movements, both natural and artificial. These seismic stations will be supplemented by thousands of others around the world that are now used to detect and study natural phenomena like earthquakes.
Atmospheric tests produce a great deal of fallout and so are relatively easily detected.
But underground tests also result in the release of some amounts of fission products into the atmosphere, enabling detection. There will be 16 laboratories to analyze the filters. Three of the four are being run by the United States. They are useful for detecting underwater tests, but also for low-altitude atmospheric tests. Four infrasound stations are currently reporting, three of which are in the US and one in Australia.