The determination of optical constants from spectra of low-symmetry single crystals or from spectra stemming from unfavorable crystal faces by dispersion analysis is a difficult task. Besides oscillator frequencies, band positions can additionally depend on oscillator strength as well as on the relative orientation of the transition moment. Therefore the results of the analysis are highly prone to errors. We present an easy method to validate the oscillator data resulting from dispersion analysis by the comparison of measured and simulated spectra of randomly oriented polycrystalline materials. Depending on the crystallite size, either average reflectance and transmittance theory (ARTT) or average refractive index theory (ARIT) can be applied to model the spectra of polycrystalline compounds. As an alternative to ARIT, effective medium theory (EMA) can also be employed. However, since principal dielectric functions do not exist in the general triclinic case, which are needed for the application of EMA, we suggest using the eigenvalues of the dielectric tensor function instead. Our method for the verification of optical constants of single crystals is validated using monoclinic CuO as an example.