Abstract: A laser beam is directed onto a light detector along a path passing close to the spray orifice(s) of a fuel-injector spray nozzle. The photoelectric signal produced by the light detector is fed to an automatic signal-content evaluating unit. To test the nozzle for proper buzzing, fuel is applied to the nozzle at a predetermined pressure and volumetric flow rate preferably corresponding to those for which the nozzle is designed. If the nozzle buzzes properly the photoelectric signal is pulsating, otherwise continuous. To test the nozzle for seal-tightness, fuel is applied to the nozzle for a limited testing interval at a pressure lower by a predetermined amount than the opening pressure of the spray nozzle. If the nozzle is insufficiently seal-tight, drops of fuel are discharged from the spray orifice(s) resulting in corresponding alteration of the photoelectric signal.