Optical-feedback laser diodes have been widely applied in chaotic optical communication, because it can simply generate wideband and high-dimension chaos and its external-cavity length is often regarded as an additional key. In this paper, it is experimentally demonstrated that the time delay signature of a chaotic laser diode with optical feedback can be identificated. Through locally enlarging the power spectral of chaotic light and observing its fine structure, the external-cavity key can be directly extracted by the spectrum analyzer, regardless of the laser diode with single or dual optical feedback of different external cavity length ratio.