This will surge the system with an unusual amount of water. The inspector can then look for signs of leakage or odors. Some inspectors will even put a dye into the system to aid in spotting leakage. If leakage is present, then the system is in failure. If an odor is noted but no leakage, this is a sign that the tanks may need to be pumped and further inspection is advised.
If your Home Inspector sells you this limited (visual only) inspection you are literally only looking at the surface. You will have no idea as to the condition of the underground components. Is the tank leaking? Are tree roots growing into the tank? What's the capacity of the tank? How much life is left in the leaching field? None of these important questions can be answered with a limited (visual only) inspection.
You should have the underground parts of the system inspected by a licensed professional septic contractor. This involves digging four holes into the ground, one at each end of the tank and two in the leaching field. Each end of the tank should have an access cover. The cover can be removed for pumping and inspection of the baffles. Visually inspecting the tank will determine its size, the condition of the two baffles and whether it needs to be pumped out. If the inspection is being performed at the time of pumping the tank, the presence of a back flow from the field can be determined.
The two holes in the field will allow an inspection of the stones. The discoloring of stones can indicate that a system is in failure or going to fail in the near future. As the field loses its ability to percolate the treated septic water into the ground, the water starts to back up into the stones. The stones will turn black below the septic water line. As this black line approaches the top of the stones the life of the leaching field shortens.
Very few Home Inspectors have the ability to pump out a septic tank to offer a complete septic inspection. Therefore, you should always have a licensed septic company pump out and inspect the system as part of your inspection of the property. It is very common to have the seller pay for the pumping of the tank. This way you can start with an empty tank and an inspected system.