On the fulfillment of a specified condition, a temperature signal outside an actuating device is detected to which a piezoelectric actuator is assigned. A piezoelectric temperature value is determined by the temperature signal. A temperature-capacitance characteristic value of the piezoelectric actuator is determined by the piezoelectric temperature value through specified mapping. A measured capacitance characteristic value is determined by a detected piezoelectric actuator charge and voltage corresponding to the temperature signal. A first correction capacitance characteristic value is determined by the measured capacitance characteristic value and the temperature-capacitance characteristic value. Independently, the charge and the voltage of the piezoelectric actuator is detected and the measured capacitance characteristic value is determined on the basis of these. The piezoelectric temperature value is determined by the measured capacitance characteristic value and the first correction capacitance characteristic value through inverse mapping with respect to the temperature and the capacitance characteristic value.