If you are buying a house it is in your best interests to have the system inspected by a professional inspector, even if you also have hired a home inspector. Often times a home inspector will assess the condition of visible plumbing in the home, but will not perform a detailed evaluation of the septic system.
Most inspections involve a dye test whereby the inspector injects a fluorescent dye into the septic system. If the dye shows up on the surface of the ground above the leach field, the septic system is not performing properly.
Another test is an "open pit test." This test involves emptying the tank and the dirt that covers the top of the tank and distribution lines. Mirrors or video cameras are then lowered into the tank so that it can be visually inspected for defects.