SEOUL, South Korea — With North Korea appearing set to detonate an atomic device, the U.N. agency that detected two previous tests says it is prepared to confirm an explosion when it takes place. But experts say it might be difficult to establish whether the blast is nuclear.

The best indication of a test will be seismic tremors and gases released into the air, phenomena that the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty identified from previous testing.

The Vienna-based organization’s most potent detection tools are more than 150 seismic stations across the globe. Although very small in yield, North Korea’s first test in 2006 was picked up by the CTBTO, as was a second test in 2009.