How does the u. s know that north korea has a h bomb

We really don't know if it was a hydrogen bomb or not. It is seriously doubted though because their are seismic detectors all over the world and the bomb wasn't as strong as it should have been. The U.S have said that the records that the bomb gave off was around a 4.8 magnitude event. A hydrogen bomb would've given off a 6.8. So their was definitely an explosion, but not as big as korea claims it to have been.