My old geology teacher said that the test-ban-treaty was a huge boon to geologists, since there was now a bottomless pit of money to be used to build seismographs anywhere you wanted to build them.
An exploding buried nuke will send energy in all directions, thus the first movement of the ground is away from the epicenter for all measurement stations around.
An earthquake usually comes about because tension between different areas release, and in that case one part of the ground will start their movement away from the epicenter, the other part will start into the direction of the epicenter.
However, one might need measurement stations covering the whole globe to detect it this way.
edit: Hm, I can't find the image that I've seen in a plate tectonics book a year ago, but here's a plot from a model:
http://www.wlandry.net/Projects/Gamra
If you look at the arrows in light blue, you can see that some point into the direction of the epicenter while some point away, thus it's an earthquake and not a nuke.
An example: http://quakesos.sosearthquakesvz.netdna-cdn.com/wp-content/u...