A device for checking a wheel of a rail car for flat spots. The device includes a microelectromechanical microphone for acquiring measured air-borne sound values within a first time span. In addition, the device includes a processing unit, which is configured to determine, as a function of the measured air-borne sound values acquired, if the wheel has a flat spot. The essence of the present invention is that the device includes an acoustic waveguide. In addition, the device takes the form of a mobile device and may be situated on or in the rail car in such a manner, that air-borne sound, which is radiated at a boundary surface, as air-borne sound, by structure-borne sound propagating through the rail car, is transmitted to the microphone by the acoustic waveguide. Also described is a related method for checking a wheel of a rail car for flat spots.