Varistors are non-linear, voltage dependent resistors. Due to this property they are used as over-voltage protection in electric circuits and electronic devices. The standard material for the manufacturing of varistors are doped ZnO ceramics. Their special electrical behavior is caused by the formation of so-called double Schottky barriers at the ZnO grain boundaries. Therefore, the determination of the electrical properties of individual grain boundaries is crucial for a deeper understanding in the behavior of varistor devices. In this work, the electrical properties of industrially manufactured varistor ceramics were investigated with regard to the microstructure. The experimental techniques employed were on the one hand variants of atomic force microscopy (AFM) and on the other hand a new micro four-point probe (M4PP) measurement setup. The utilized AFM based methods were conductive atomic force microscopy (C-AFM), Kelvin probe force microscopy (KPFM), scanning surface potential microscopy (SSPM), and scanning impedance microscopy (SIM). The AFM investigations revealed a high conductivity of the ZnO grain interior and a high resistivity of the grain boundaries. KPFM and SSPM allowed to identify a specific grain boundary with reduced resistance causing a preferred current path, which was previously found by thermography measurements. Furthermore, it was possible to detect asymmetric behavior for current paths across two grain boundaries. The M4PP measurements allowed to investigate the current to voltage characteristics of individual grain boundaries, at which asymmetries and strong variations in the parameters - most important for the device performance - were found.