We report a combined experimental and computational study on the heterogeneous electron transfer kinetics for a simple one electron transfer reaction (ferrocene/ferrocenium Fc+/Fc couple) in a series of molecular solvents and ionic liquids. We focus on the diagnostics of the electron transfer regime (adiabatic vs nonadiabatic) and assess the parameters of the quantum mechanical electron transfer theory, which determine the observed tendencies in the solvent effect on the electron transfer rates. The applicability of the linear plots of the electron transfer rate constant vs longitudinal relaxation time (or solvent viscosity) for distinguishing between different ET kinetic regimes is analyzed. Classical molecular dynamics simulations were performed to calculate the potential of mean force for Fc and Fc+. The structure of the reaction layer derived from molecular dynamics is thoroughly investigated. The experimental dielectric spectra for the both type of solvents are used for quantum corrections of the outer-sphere reorganization energy as well as for estimations of the effective frequency factor in the limit of strong and weak electronic coupling. The electron transfer rate constants are calculated and discussed in the viewpoint of available experimental data.