To determine if a pulsed beam system, typically an ultrasonic pulsed beam system, which is located externally of a motor vehicle is in operative condition, a transmitter (14) projects a beam to a fixed reflector (16) located on the vehicle, and one (19) of a group of timing stages (18; 19, 20, 21) evaluates the transit time to the reflector. If the transit time to the reflector is slightly less than a predetermined time interval (Tc), so that the threshold timing level (Tc) is passed, a first indication, for example a frequency at a first tone (f1), generated by an acoustic tone generator (30) is reproduced in a loudspeaker; if, then, an obstruction is sensed by the beam, and the distance between the transmitter (14) and the obstruction (17) which forms a reflecting object drops below a first predetermined distance represented by a time period (T2), a second one of the transit time stages responds, and, through logic gates (22-24, 28, 29) provides a second discrete output frequency (f2) in the loudspeaker. If the vehicle closely approaches the reflecting object or obstruction (17), a third frequency (f3) is reproduced by the loudspeaker (26) and, additionally, a warning signal can be given. The system can be connected to the reverse gear or rear light switch, so that, if no obstruction is sensed, and only the signal from the reflector is reproduced, an audible indication of "reverse gear engaged" is provided, thus simultaneously providing a reverse gear signal as well as a signal representative of proper operation of the system. To compensate for ambient condition, dirt on the transmitters, and the like, the timing intervals by the transit time stages responding to the obstruction can be changed (FIG. 2) by providing a calibrating input based on actual transit time from the transmitter (14) to the fixed reflector (16).