An actuator of a machine is controlled by a control device with a controller structure in a normal operating mode and a special operating mode. The control device determines from a position setpoint value and a position actual value a control signal for the actuator. In the normal operating mode, the setpoint values are determined using a utility program, and in the special operating mode by a system program that is different from the normal operating program. In the special operating mode, a frequency characteristic for the actuator is determined on the basis of the sequence of setpoint values and associated actual values, and an evaluation for the actuator and/or of the controller structure is performed using the frequency characteristic and parameters of the controller structure. Depending on the evaluation, a message is transmitted to an operator of the machine or to a computing device over a network.