Simple Tests to the Wall insulation

Checking the wall insulation may be less straightforward and less immediate than auditing the attic insulation, which is largely based on direct observation.

Checking through a small hole in the wall

To check a wall insulation, you can make a small hole in a closet, or in some hidden and unobtrusive place, to inspect the wall cavity and see if it is filled with some sort of insulation, and its type and amount. You may use a plastic crochet hook or something equivalent to check for the wall insulation.

In the case of solid walls you may remove one (or a few) small sections of the exterior siding, reconstituting it later, after the inspection.

Checking through outlets in the wall

You may also inspect the wall and its cavities through the outlets, instead of making a hole. Just make sure that outlets are not getting any electricity. Test the outlets to make sure they aren’t switched on. Be careful.

After selecting an exterior wall and an outlet in it, remove the cover plate from the outlet and probe into the wall. You may then use a screwdriver or a sufficiently long and thin stick...

Infrared video checking

Checking a wall insulation through the means suggest above has obvious limitations.

Such tests to not ensure that the insulation is applied in the entire wall, or that it is well settled and without defects. To get that information you need a professional audit, usually based on infrared video and cameras (these tools are part of what it is technically known as thermographic auditing).

See:

Professional energy home audits
Thermographic Inspections