In a method for testing a bar winding of a rotor of a rotating electrical machine, a rotor temperature is detected by a thermal radiation sensor, which is arranged in a stator of the rotating electrical machine. A thermal radiation sensor is used with a resolution suitable for the thermal detection of individual bars of the bar winding. The rotor is rotated at a speed that is lower than a limit frequency of the thermal radiation sensor divided by the number of bars of the bar winding, and the thermal radiation of the rotor is detected by the thermal radiation sensor. The thermal radiation sensor is sampled at a sampling frequency that is higher than the speed of the rotor multiplied by the number of bars and the sampled values of the thermal radiation sensor are evaluated.