Soldiers and vehicles are fitted with infrared laser detectors for detecting simulated laser bullets and artillery shells from SAT-equipped small arms weapons and tanks employing laser scanner transmitters. An infrared LED is mounted inside the protective housing which supports each laser detector for illuminating the exterior surface of a window, lens or other transparent optical element positioned in front of the detector. Dirt, dust, mud, snow, shoe polish or other contaminant on the exterior surface of the optical element scatters the infrared radiation from the LED and in accordance with a test periodically performed by a test circuit, if the scattered signal exceeds a predetermined threshold value, a visual and/or audible warning is given to the player. If the optical element is not cleaned within a predetermined time after the warning, a kill command is executed to prevent the player from cheating.