Selective electron cyclotron emission and absorption by fast electron tails are discussed by using the concepts of momentum space spectra and collective resonance velocity. This is defined from the maximum of the emission and absorption profiles in the momentum space for a system of electrons and represents the counterpart of the one-particle resonance velocity. It is shown that the collective resonance is sensitive to the superthermal momentum distribution and is, therefore, a useful parameter in the diagnosis of the fast electron tail. A simple relation between the collective resonance and the emission and absorption coefficients for propagation normal to the magnetic field is found. This theoretical framework is used for a new diagnostic method based on the localized (in phase space) perturbation of the electron tail by an electron cyclotron wave and emission and transmission of radiation at higher harmonics. A numerical application is presented for the case of a lower hybrid sustained tail, using the appropriate Fokker-Planck and ray tracing codes.