[klodolph]

And the fireball created by the X-rays is, for a moment, hidden behind the shock wave. This makes a "double flash" which can be detected from orbit without even the need for a camera, just point a photodetector at the Earth's surface.

[arethuza]

Bhangmeters are one kind of specialised photo detector used to detect nuclear explosions:

http://en.wikipedia.org/wiki/Bhangmeter

I'm pretty sure these were used in bunkers (at least in the UK) as well as satellites.