USGS indicated no seismic activity for the last 48 hours, according to Fox report.
Secretary Rumsfeld was asked, "would we know?". He said we would know. This from Bret Baer, Fox.
But they also said that it might not register on the USGS's sensors, so no reading isn't necessairly indicative of no test. Of course, there are other ways to tell if a test has been conducted and I'm sure our intelligence agents are looking into those right now.