A system for identifying sources of external fluid infiltrating or leaking into a sanitary sewer system. An apparatus applies a substantially uniform coating to the surface of inner walls of a portion of the sewer system. The system also includes means for observing and recording the initial condition of the coated surface, means for observing and recording subsequent conditions of the coated surface, and means for comparing the initial condition of the coated surface with subsequent conditions to detect a predetermined change in the condition of the coated surface. The change in the condition of the coated surface indicates the source of the infiltration of water into the sewer system.