Vent pipes terminating at the roof should all be inspected as well. These vent pipes allow air and pressure to enter/leave so water can drain properly. If there is an issue with venting, sinks, showers and toilets would have problems draining properly. In this quick video, I am inspecting an older home with cast iron drain piping. As I look into the pipe, you should just see the inside of a pipe as far as you can see until it makes a turn. This is a good place to see if there are any obstructions etc. Surprisingly, I found this pipe was cut below the roof line. I confirmed this in the attic.