Biohybrid hydrogels combining electrically neutral synthetic polymers and highly anionic glycosaminoglycans (GAGs) offer exciting options for regenerative therapies as they allow for the electrostatic conjugation of various growth factors. Unraveling details of ionization and structure within such networks defines an important analytical challenge that requires the extension of current methodologies. Here, we present a mean-field approach to quantify the density of ionizable groups, GAG concentration, and cross-linking degree of such hydrogels based on experimental data from microslit electrokinetics and ellipsometry. An exemplary poly(ethylene glycol)-heparin system was analyzed to demonstrate how electrostatic fingerprints of hydrogels obtained by the introduced strategy can sensitively display composition and structure of the polymer networks.