Simple: Look at the impedance behavior of the dielectric, e.g. consider the frequency dependence of the phase shift between AC current and voltage for the dielectric being put into a capacitor (or look at the transmission of microwave radiation or whatever). Due to the polarization mechanisms involved, in some frequency ranges even a perfect dielectric shows absorption. (Without polarizability, it would not be a dielectric!) For the details, see the relevant parts of the lecture "Electronic Materials" by Prof. H. Föll, University of Kiel (http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_3/backbone/r3_3_1.html and http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_3/backbone/r3_4_1.html).

Thanks a lot.