Interphases are commonly believed to be responsible for the physical properties of nanodielectrics unexplainable by general mixture laws. For instance, the prediction of the dielectric permittivity of these materials supposes to include the intrinsic permittivity and the volume of the interphase. Despite the need for a local characterization of these nanometric regions, no sufficiently reliable proof of its existence has been reported yet. Electrostatic Force Microscopy (EFM) constitutes a promising technique to accomplish this objective. However, the study of interphases with EFM has never been completely and accurately established yet, and the interpretation of EFM signals can be misleading. The aim of this work is to set up a first accurate method and signal analysis to examine interphases in nanodielectrics. Model samples of sufficiently known and mastered properties have been designed and synthesized to "calibrate" the technique for future interphase study. EFM proved good sensitivity to thin alumina interphase model layers between polystyrene spheres and polyvinyl acetate matrix. Consistent permittivity values have been also deduced after confrontation to numerical simulations.