Satellite photos?

You can easily get an estimate of the number of buildings and especially vehicles, which tell you two important things. Not to mention that as a matter of course the first thing to do is photograph everything that looks like a piece of military equipment, which has been a purpose of satellite photography from the beginning.

Various kinds of countries get paranoid about letting people have maps or accurate geographic data. This makes very little difference militarily but causes real inconvenience for the locals.

Besides, nobody wages wars for labour exploitation any more. It's all about what's under the ground.

Yeah, spy sat tech today is likely good enough to track every single person that steps outside in the FoV of the sat in real time.