The time to ook (ok, to look) for them would be when it happened. We have radioastronomers looking at stuff billions of miles away, why wouldn't this show up on their instruments too.
No doubt due to the Earth being a sphere and not being flat. Radio Telescopes in the US are generally pointing away from North Korea. If this was a real nuke, we should have had confirmation by now from all the sensors we have pointed right at North Korea (not radio astronomer equipment, but DOD equipment specifically designed for picking up the trace radiation). If this was the real deal we would have a blockade starting now followed a few days later with a successful Japanese test. If it was a real test Kim would not have to demand bribes. They would be freely given to keep him from setting off another.
Tests can be hidden. That's why the Test Ban Treaty is and always has been a complete joke. And we are suckers to have signed it.