Abstract: The invention is directed to a sensor apparatus for determining when a water outlet needs flushing, comprising a water flow monitoring sensor adapted to be in contact with a source of water flowing through a conduit; a microprocessor in electronic communication with the water flow monitoring sensor, the microprocessor configured to calculate whether the water stagnation time exceeds a predetermined threshold; and indicia for signaling when the water needs flushing by a user, the indicia n electronic communication with the microprocessor. The water flow monitoring sensor may include an accelerometer for detecting water flow vibrations typical of water flow in water conduits. A microprocessor calculates and tracks the total flush time since the prior flush of a drinking water source, thereby monitoring its safety for users of the water source.