According to the method a temperature module is defined, simulating the thermal behavior in the electric motor (1), based on a multi-mass model. The rotational speed (n) and the current (i) for the electric motor (1) are continuously recorded as input parameters for the temperature model and at least one bearing power loss and/or bearing inner temperature (TIL1′), corresponding to a bearing (8,9), is continuously derived from the temperature model. A first warning signal (W1) is given when a computer-determined bearing power loss exceeds a given comparative value. Furthermore, a second warning signal (W2) is given when a computer-determined bearing inner ring temperature exceeds a given temperature difference (ΔT) from a corresponding computed bearing outer ring temperature (TAL1′). The thermal behavior of non-accessible components, in particular, the bearing (8, 9), can thus advantageously be determined by mere evaluation of the current (i) and the rotational speed (n). The fitting of temperature sensors (2, 3, 4, 12) in the electric motor (1) can also be avoided.