Although sewer inspections aren't part of a normal house inspection, if you are buying an older home. it is worth the expense.
A sewer scan is done by a company that specializes in this service. A technician accesses the main sewer cleanout hole in the basement and feeds a fiber-optic camera through the sewer line that runs from the house to the city sewer at the street. This sewer line belongs to the property owner and any problems with this sewer line are the financial responsibility of the owner. If there are problems with the sewer line that routine sewer cleaning won’t solve, the repairs will be expensive.
In my experience, most sewer scans on an old home reveal tree roots finding their way into the sewer lines and slowing things down, but no major problems. It’s the situations where major problems occur that make it worth having a sewer scan done.
Sewer scans typically cost less than $200, and are usually scheduled at the same time as the home inspection.
Source: Excerpt from Money/Real Estate - Reuben Saltzman, Structure Tech