In an hydraulic actuator, a movement of an actuating element of the actuator is effected in that a working chamber of the actuator, with the aid of a valve device, is able to be selectively connected to, and disconnected from, a fluid reservoir in which pressurized hydraulic fluid is stored. The lift of the actuating element of the actuator is a function of a fluid volume present in the working chamber. It is provided that, to ascertain an instantaneous operating performance of the actuator, the working chamber is briefly connected to the fluid reservoir, the corresponding pressure drop in the fluid reservoir is recorded, and the corresponding lift is determined from the pressure drop with the aid of known geometrical variables of the actuator.