This paper describes a novel sensorless identification method for mechanical two-mass-systems and the influence of varying parameters and operation conditions on its performance. The identification procedure is carried out during the encoderless control of the drive system and based on the frequency response measurement of the mechanical setup. An extended speed adaptive observer structure serves as speed estimate for the calculation of the Bode diagram. The extension of the observer structure for speed estimation by a mechanical model allows a reliable identification of various two-inertia-systems within two identification steps. This paper gives an overview of the new identification procedure and the results of investigations toward its robustness and limitations. Varying speed and load conditions as well as different parameterizations of the signal processing method underline the applicability of the presented method.