North Korea’s Punggye-ri Nuclear Test Site: Significant Acceleration in Excavation Activity; No Test Indicators but significant increase in soil excavated by comparing size of "Dec 2013 Spoil Addition" (frame on left) with "Jan 2014 Spoil Addition" (frame on right).
It can be concluded that North Korea will conduct its fourth nuclear test in 2014 and probably from late April 2014 - based on data provided by Johns Hopkins University's US-Korea Institute. The US has a wide range of intelligence platforms to predict the (virtually exact) timing of the fourth test - including satellites (of course) and RQ-170 Sentinel stealth drones deployed in the region.
Agency France Press (AFP) via Australia's 7 News, reported, February 14, 2014 http://au.news.yahoo.com/world/a/21464288/excavation-work-at-n-korea-nuclear-test-site-us-think-tank-claims/ :
"Excavation work at N.Korea nuclear test site, US think-tank claims
Seoul (AFP) - Recent satellite imagery suggests North Korea has stepped up excavation work at its main nuclear test site, but there are no signs of an imminent test, a US think-tank said on Friday.