We present a combined study of resonant Raman scattering (RRS) from LO phonons and electroreflectance (ER) modulation spectroscopy, to analyse internal electric fields in ZnSe-based heterostructures. While modulation spectroscopy is commonly applied and evaluated quantitatively by using Franz-Keldysh oscillations, up to now much less detailed studies of electric field effects on Raman spectra are available. We report on field dependent changes in the near resonance Raman cross section, extending beyond a decade, which is in strong contrast to the promille range effect, which is observed for electroreflectance. The field dependent resonance can be described by Frohlich-induced Raman scattering with electronic intermediate states according to the Franz-Keldysh theory. This allows a quantitative determination of electric fields by Raman spectroscopy, which offers an enhanced layer selectivity in complex heterostructures because of the unambiguous identification of the contributions of each material due to its specific phonon frequency. Beside the comparison with RRS, the ER on ZnSe is applied to study the dependence of the internal field on the metal electrode material (here: Au; Al). From these results the Schottky barriers are derived, yielding a difference of 0.8 eV between ZnSe/Au and ZnSe/Al.