A method for off-line testing of an electric motor is disclosed. The motor having at least one stator winding, and a rotor arranged along a rotation axis. The method includes applying a periodic test signal to the at least one stator winding; collecting first measuring data, related to a physical quantity of the at least one stator winding while the rotor is being rotated about the rotation axis, from one or more waveform periods of the test signal, detecting first peak values of the first measuring data; forming second measuring data based on the first detected peak values, detecting second peak values of the second measuring data for the at least one stator winding; determining a mutual relationship between the at least a portion of the second peak values; and, providing, if the mutual relationship deviates from a predetermined relationship, a signal indicating a fault in the rotor. Also disclosed is an apparatus for performing the disclosed methods.