We have undertaken polarization-resolved SHS experiments to investigate a series of two fluorescent organic nanoparticles (FONs) made from dipolar dyes bearing a triphenylamine-donating end-group and a slightly hydrophilic acceptor end-group. The FONs, which show very large negative surface potentials, responsible for their good colloidal stability in water, are very bright SHS emitters in water, attractive for single-particle tracking and bioimaging. With the polarization-resolved SHS technique, we come up with a new and noninvasive method to gain insight into the origin of the surface potential of the FONs, as well as into their evolution upon change of ionic strength by following their multipolar response when adding salt. In pure water, the polarization-resolved SHS spectroscopy reveals the presence of a region of local polar order close to the surface (i.e., orientation of the dipolar dyes), leading to a strong dipolar contribution. Quite interestingly, when adding KCl salt, the initial two-lobe SHS polar plot transforms into a four-lobe pattern with a net decrease of the SHS intensity, revealing a lower surface potential. Second, we have quantified the decrease of the SHS signal when adding salt with the goal to estimate the change of the NPs' colloidal stability in salty media. From a simple equilibrium model that we propose, we have extracted a lower bound free energy for the two FONs, which are thermally accessible and in good accordance with their respective colloidal stability.