We report on optical levitation experiments to probe the interaction of a nanoparticle with a surface in vacuum. The observed interaction-induced effect is a controllable anharmonicity of the particle trapping potential. We reconstruct the Coulomb image charge interaction potential to be in perfect agreement with the experimental data for a particle carrying elementary charges and compare the measured electrostatic interaction with the weaker dispersive forces from theory. Our experimental results may open the route for a new surface sensitive scanning probe technique based on the high mechanical sensitivity of levitated nanoparticles.