How Do We Know Our Nuclear Weapons Will Even Work?

Bombs away? Or Duds Away? How Old Is This Stuff?

Image by Gerd Altmann from Pixabay

I barely passed high school physics, so I’m certainly not qualified to answer my own question.

I’m just wondering if the huge planet destroying nuclear stockpiles in the US, Russia, N. Korea, China to name the biggies are still capable of taking out whole cities?