In this paper we present a technique allowing to identify harmonic transponders. The approach relies on the evolution of the backscattered power at a given harmonic frequency as a function of the fundamental frequency. Thus to identify an harmonic transponder, the associated reader sweeps a continuous wave in the considered bandwidth. Experiments are done based on a vector network analyzer, the method is able to identify two different RECCO reflectors. A study on the read range is also included. These harmonic transponders allow to drastically increase the read range compared to the one associated to linear timeinvariant systems such as chipless tags.