You can a couple of things with that, you can do a visual inspection, and note that on your report.
There is no way for you to "see" what or how the wiring is behind walls. If the company wants you to look behind every device and J-box, it would cost them more for labor involved.
Most of all leave yourself a "way out" on your report. Most businesses do this. They leave in a "grey" area. Something like:
This inspection was limited to, ect.....
As for companys who do this, Burlington Electrical Testing is quite big and does this kind of work, and charge a ton for it. Once you see what they do, you'll want to do it, very good money involved, very little time spent.