Vibrational sum-frequency generation (SFG) spectroscopy is demonstrated as a fast method to quantify variations of the electric double-layer potential Φ₀ at liquid/gas interfaces. For that, mixed solutions of nonionic tetraethyleneglycol-monodecylether (C₁₀E₄) and cationic hexadecyltrimethylammonium bromide (C₁₆TAB) surfactants were investigated using SFG spectroscopy and a thin-film-pressure-balance (TFPB). DLVO analysis of disjoining pressure isotherms obtained with the TFPB technique provides complementary information on Φ₀, which we use to validate the results from SFG spectroscopy. By using a single Φ₀-value we can disentangle χ⁽²⁾ and χ⁽³⁾ contributions to the O-H stretching modes of interfacial water molecules in the SFG spectra. Having established the latter, we show that unknown double-layer potentials at the liquid-gas interface of solutions with different C₁₆TAB/ C₁₀E₄ mixing ratios can be obtained from an analysis of SFG spectra and are in excellent agreement with the complementary results from the TFPB technique.