Yikes! A backed up sewer line is one of the worst nightmares of homeowners. How do you know if your sewer line might be backing up, and it's not just a routine plumbing problem you are facing? A Seattle plumber posted some tips for identifying when your main sewer line is backing up. Signs include: muddy brown water coming up from a drain, a bubbling toilet when not in use, and water pooling on the floors near bathroom fixtures. Sewage coming up in your yard is a definite sign that you might need to auger your sewer line.