Yes, there’s no doubt. They’ve tested multiple devices, several of them failed but the 2017 one didn’t. We know they bought the designs and materials from Pakistan, we’ve seen enough images of the process and equipment to know they were actively making progress, and we can measure the size of explosions via seismographs. We can measure the craters left behind via satellite images.

The wiki article has a good rundown of the evidence: https://en.m.wikipedia.org/wiki/2017_North_Korean_nuclear_te...