A sensor module includes a window structure configured to permit the passage of transmitted light and received light between an inside of the sensor module and a field-of-view; a transmitter configured to transmit a transmit light beam; a scanning structure configured to rotate about at least one scanning axis, the scanning structure configured to receive the transmit light beam from the transmitter and direct the transmit light beam towards the window structure and the field-of-view; a light detector configured to detect a reflected light beam that corresponds to the transmit light beam; and at least one processor configured to measure a time-of-flight of a round trip light beam comprising of the transmit light beam and the reflected light beam, compare the time-of-flight to a threshold time, and detect a dirt formation on the window structure on a condition that the time-of-flight is less than the threshold time.