If you sense that there is a problem with your septic system, there are some early warning signs to look for that could keep you from having a major disaster on your hands. If you are experiencing any of the following signs, contact your local health agency for assistance:
• Odors, surfacing sewage, wet spots, or lush vegetation growth in the drainfield
• Plumbing or septic tank backup
• Slow draining fixtures
• Gurgling sounds in the plumbing system
References
http://www3.doh.wa.gov/here/materials/PDFs/24_SepTank_E04L.pdf
http://people.howstuffworks.com/sewer.htm