A novel method for electrostatic potential measurements at the interface is described. It involves placing a two-dimensional grid below the sample and observing it in a scanning electron microscope. Primary electron beam displacement, caused by surface charges, can be then measured for every grid knot. Using geometric parameters of the setup, a quantitative mapping of the potential can be extracted. It is shown that this method can achieve a tens of millivolt sensitivity and a submicron spatial resolution in electrostatic potential measurements.