  A method allowing analysis of the overpopulation of phase-space in heavy ion
collisions in a model independent way is proposed within the hydrodynamic
approach. It makes it possible to extract a chemical potential of thermal pions
at freeze out irrespective of the form of freeze-out (isothermal) hypersurface
in Minkowski space and transverse flows on it. The contributions of resonance
(with masses up to 2 GeV) decays to spectra, interferometry volumes and
phase-space densities are calculated and discussed in detail. The estimates of
average phase-space densities and chemical potentials of thermal pions are
obtained for SPS and RHIC energies. They demonstrate that multibosonic
phenomena at those energies might be considered as a correction factor rather
than as a significant physical effect. The analysis of the evolution of the
pion average phase-space density in chemically frozen hadron systems shows that
it is almost constant or slightly increases with time while the particle
density and phase-space density at each space point drops down rapidly during
the system's expansion. We found that, unlike the particle density, the average
phase-space density has no direct link to the freeze-out criterion and final
thermodynamic parameters, being connected rather to the initial phase-space
density of hadronic matter formed in relativistic nucleus-nucleus collisions.
