The structure of the polarised interface between two immiscible electrolyte solutions (ITIES) has usually been deduced from electrocapillarity and capacitance measurements. However, these two methods only offer an indirect access to the surface change density, by differentiation and integration. In this work, we analyse the dependence on the potential difference and the organic phase electrolyte concentration of the surface charge density of the polarised ITIES. Our study is based on electrocapillarity, capacitance and surface second harmonic generation (SHG). This spectroscopic approach allows us to probe directly ionic concentrations and hence surface charge density. By comparing the results provided by these different methods, we confirm that integration of the capacitance curves yields too large values of the charge density. On the other hand, electrocapillarity and SHG provide consistent results over a large potential range. These results depict an ITIES weakly affected by the organic electrolyte concentration. Furthermore, the surface charge density depends quasi-linearly on the potential difference, at all concentrations. Altogether, these results support the model of a polarised ITIES made of two ionic face-to-face layers and devoid of diffuse layer, the so-called discrete Helmholtz model.