A buyer will pay to have the septic tested. As a realtor, I help to provide the names of potential inspection companies, but it is the buyer that picks, arranges the inspection, and meets with the inspector. But, keep in mind there is the cheap method and the expensive method. The cheap method is a chemical test they run. The expensive method removes the lid and inspects more thoroughly. I sizably higher expense, and few buyers typically make the extra effort or cost. I recommend you talk to the inspection companies to understand the pros and cons depending upon the age of the house and the records that exist.