The inspection I do is to measure the scum and sludge layers using a core sampler. In the last month or two there are two threads where I posted a video that explains everything about how the tanks work and how to take a core sampler. The guy who made the video sells the core samplers for $100.
I had to make my own because my tanks are so deep -- 16 feet from the pipe opening to the bottom. It only took about an hour to make and cost about $80. I explain it and have a photo of it in one of the other threads.
Bottom line: don't pump until you have taken a core sample and take core samples every 6 months or so -- having a pumping scheduled is pumping blind.
Also having a guy pump it without taking a core sample might hide the fact that the old owner was not pumping when he should and let the sludge layer build up to the point where it was getting into the drain fields -- not good, and is something that should effect the sales price.
The other test that I know about is using a tape measurer (from the dollar store) to measure the distance from the top of the out let pipe access to the water level. Then put the tape down into the water until you can hook it on the bottom of the outlet pipe. It should be about 17" below the water level, as I remember -- better check with the county inspector or a tank manufacturer.
If you can't find the old posts about septic tanks let me know and I will dig up the info again.