During a home inspection our inspectors run a lot of water at multiple fixtures to do what we call a hydro test of the plumbing system. Even with our thorough inspection we are still not able to see the interior of the sewer line. ( This is why the underground sewer lines are excluded from all professional home inspection standards and by all home inspection companies ) We also are not flushing toilet paper, doing laundry or washing food down the garbage disposal which can get caught on tree roots or deteriorated drain lines and cause a backup of the sewer line into your newly purchased home. To see these underground drain lines you need a sewer scope inspection.