We have seismographs and other measurement & signature intelligence collection means all over the world, and there's an elite unit in the US Air Force dedicated to detecting and localizing nuclear detonations no matter where they occur (as well as event attribution, nuclear forensics, intelligence gathering, etc.).

https://www.airforcetimes.com/news/your-air-force/2019/04/21...

https://en.wikipedia.org/wiki/Air_Force_Technical_Applicatio...