This paper presents for the first time a method to measure the capacitance variation of electrostatic vibration energy harvesters (e-VEHs) that employ conditioning circuits implementing a biasing scheme that can be represented by a rectangular charge-voltage diagram. Given the increasing number of e-VEHs using such complex conditioning circuits and the complex dynamics that are induced from this type of biasing, a mean to assess this measurement is of primary importance for the analysis of e-VEHs. The proposed method is based on the inspection of the voltage evolution across two simple conditioning circuits implementing a rectangular charge-voltage diagrams biasing scheme. After the method is presented, it is carried out for the characterization of a state-of-the-art MEMS e-VEH.