There really aren’t many different methods or techniques for detecting signs of missing insulation or leaks behind walls.
Infrared thermal imaging is as close as you will get. If the IR camera is in the hands of an experienced operator, then your probabilities of tracking the source of an issue behind the dry wall increases. It’s important to note that when an IR camera is pointed at your wall or ceiling, it will only tell you if there is a difference in the surface temperature (thermal anomaly). If the temperature does show up to be ‘uneven’ through the infrared camera detector, it’s usually moisture, missing insulation or a leak.
How do you conclude what the issue is?
Simple… testing with moisture confirmation equipment. If it’s in the ceiling then you can also climb into the ceiling space (if it’s not a flat roof leak), otherwise if it’s behind the wall then you have minimal options outside of thermal imaging.
One of the primary reasons for calling your local thermographer first to detect anything you suspect to be an issue in your home, is they can usually run the thermal imaging equipment over the rest of your home for a few extra dollars (as opposed to getting just one room imaged).
Here’s what missing insulation in a ceiling looks like using IR infrared…