Oregon State University geophysicist Paul Vincent recently published a research paper detailing a new way to measure underground nuclear weapons tests with satellite imagery.
Vincent's study, published in the journal Geophysical Research Letters, focused on an underground Chinese nuclear testing site. Satellite radar images revealed a two-inch surface "uplift," caused by heated groundwater from the nuke tests.
Interestingly, the earth didn't move until a full four years after the detonations. That limits the usefulness of Vincent's technique for spotting illicit nuke tests within a meaningful time frame, but it does give intelligence agencies and nuclear monitoring groups a new way to gauge past nuclear tests.
More details are available at OSU's website. If you're feeling especially ambitious, browse a PDF of the study, "Anomalous transient uplift observed at the Lop Nor, China nuclear test site using satellite radar interferometry time-series analysis," here.