Scanning Probe Microscopy with conductive tips has been used to image and study the dielectric properties of giant permittivity CaCu3Ti4O12 ceramics at the nanoscale. Since measurements are generally carried out on sections of a sample, particular attention has been devoted to possible artefacts due to surface imperfections, such as substantial surface roughness and/or contamination that can result in controversial interpretation, particularly at nanometric spatial dimensions. A reliable surface investigation has been carried out after the definition of both the physical and geometrical unbiased criteria to avoid any artefacts due to surface roughness and/or anomalous tip-sample contact variations. The presence of insulating grain boundaries and the measurement of a depletion layer at the grain-grain boundary interfaces unambiguously demonstrate the relevance of the Internal Barrier Layer Capacitor effect, among all the proposed physical mechanisms, to explain the giant dielectric behaviour. Such imaging provided a clear correlation between the macroscopic dielectric properties and the nanometric structure at the interfaces. Moreover, the "general criteria" for reliable nanoelectrical characterization as well as the related measurement resolution have been defined.