An insulation contractor told me he goes into a bedroom closet, pokes a little tiny hole in the exterior wall at the bottom corner just inside the door, then sticks a little bent wire in there and rotates it like a pinwheel. That’s how he finds missing insulation.
I’ve been removing cover plates and that usually works, but sometimes you have to cut the drywall back a little and it’s messy. I’ll bet a little wire with the end bent onto a hook would work better at outlets.
I wonder if someone were to hold an electronic guitar tuner next to the wall while tapping the wall, if empty cavities would sound a different note from full cavities. Hey! An excuse to have another electronic gizmo!