It is indeed how you write the contract. Have your lawyer stipulate that one of the conditions of sale is that the septic tank needs to be inspected by a reputable company. It must pass, and you need to get the inspection report in writing.
This can save you thousands down the line. Should the tank fail the inspection, you can use that to either renegotiate for a lower sale price or as a means to walk away from the deal.
The health dept. inspects & certifies the septic-give them a call. Also check to see if it was done and when by a licensed contractor (in my area do it yourself is allowed by homeowner) I had one that was bad. The owner should have a pumpout card to show when it was last cleaned out. Estimate your usage (family size etc) & call local septic service provider to verify next pump out date & cost. Often here new buyer negotiates pumpout cost out of home price.
I have decided to move on to a Public Sewer unit. The Septic unit that I once interested was purchased by someone else at full price. There was no room to neigociate. but I will keep the knowledge in mind for the next septic deal.