We report the results of second-harmonic optical spectroscopy of LaAlO3 films grown on SrTiO3 (001) substrates having either TiO2 or SrO atomic terminations. The atomic termination is found to have a large effect on the observed spectra, with a strong dependence on the ω-2ω light polarizations. These results are analyzed through a model based on symmetry-controlled selection rules and atomic orbital overlaps. The analysis shows that, as suspected from transport measurements, the SrO termination prevents the formation of a mobile interfacial electron gas. Yet, by explicitly probing and identifying the electronic states at the O2−–Ti3+ band gap we now obtain unique and direct evidence that this is caused by the absence of any detectable charge injection in the interfacial region rather than by carrier localization.