We have geosynchronous infrared satellites over the area, and they are primed to watch 24/7 for thermal blooms associated with missile launches. We are alerted within seconds of such, as well as indications of fueling of liquid-fueled multi-stage rockets. Only the short-range rockets can be fired without alerting us in advance, and even these are caught on launch.
I don't think NK has the ability to deliver a bomb in North America unless they used a plane, in which case I don't think that plane would make it very far. I wouldn't worry about what NK can do by themselves. To me, the worry is in the country's that back both of the Korea's (US-South Korea and China-North Korea).
first of all pl note it is not korea but north korea-time taken for ballistic missile to travel from n korea will be around 20 minutes and by that time this missile will be neutralized and N KOREA will vanish from world map