We probe local charge fluctuations in a semiconductor via laser spectroscopy on a nearby self-assembled quantum dot. We demonstrate that the quantum dot is sensitive to changes in the local environment at the single-charge level. By controlling the charge state of localized defects, we are able to infer the distance of the defects from the quantum dot with $\pm 5\mathrm{nm}$ resolution. The results identify and quantify the main source of charge noise in the commonly used optical field-effect devices.