Electrostatic force microscopy at cryogenic temperatures is used to probe the electrostatic interaction of a conductive atomic force microscopy tip and electronic charges trapped in localized states in an insulating layer on a semiconductor. Measurement of the frequency shift of the cantilever as a function of tip-sample bias voltage shows discrete peaks at certain voltages when the tip is located near trap centres. These discrete changes in frequency are attributed to one by one filling of individual electronic states when the quantized energies traverse the substrate conduction band Fermi energy as the tip-sample voltage is increased. Theoretical analysis of the experiment suggests that such a measurement of the cantilever frequency shift as a function of bias voltage can be interpreted as an AC force measurement, from which spectroscopic information about the location and energy of localized states can be deduced. Experimental results from the study of a sample with InAs quantum dots as trap centres are presented.