This paper concerns the use of Frequency Modulated Continuous Wave (FMCW) radars for Radio Frequency Identification (RFID) transponders identification and range local-ization. In our study, we consider the transponder identification by applying a shift frequency to the RFID transmitted signal. Consequently, our investigation addresses the spectral analysis of the received signal on the FMCW radar. First, the classical FFT method is used for detection of two or more RFID transponders which could be tagged by near shift frequencies. Then, spectral estimation and high resolution methods are evaluated to enhance the detection and discrimination of transponders both in case of free space or Rayleigh fading propagation channel. Finally, the impact of non-linear frequency sweep on the performance of FMCW radar processing is examined. Modeled results are presented showing the effect of such non-linearities on detection accuracies of the transponder.