Magnetotunneling spectroscopy of resonant tunneling structures provides information on the nature of the two-dimensional electron gas in the well. We describe a model based on nonequilibrium Green's functions that allows for a comprehensive study of the density of states, tunneling currents and current spin polarization. The investigated effects include the electron - phonon interaction, interface roughness scattering, Zeeman effect and the Rashba spin - orbit interaction. A qualitative agreement with experimental data is found regarding the satellite peaks. The spin polarization is predicted to be >10% for magnetic fields above 2 T and having a structure even at the satellite peaks. The Rashba effect is confirmed to be observable as a beating pattern in the density of states but found to be too small to affect the tunneling current.