A method determines an injection law of a fuel injector to be tested in an injection system and includes steps of: completely interrupting feeding of fuel from a fuel pump to a common rail; avoiding opening of fuel injectors except for one injector to be tested; measuring initial pressure of the fuel inside the rail before starting the opening of the injector; opening the injector for consecutive openings greater than one with a same test-actuation time; measuring final pressure of the fuel inside the rail after ending the opening of the injector; determining a pressure drop in the rail during the opening of the injector, which is equal to a difference between the initial and final pressures; and estimating, according to the pressure drop in the rail, a fuel quantity that is actually injected by the injector when the injector is opened for the test-actuation time.