In this work, we present a methodology to investigate the evolution of drop size distribution, due to coalescence, in liquid-liquid suspensions under simple shear flow. Our approach is based on 3D motorised sample scanning by optical sectioning and offline analysis of the acquired images. The application of such automated procedures allows the detection of a large number of drops, as required to minimise sampling errors and to obtain reliable results according to statistical methods from the literature. Three-dimensional reconstruction of the locations of the drops within the sample allows to evaluate possible wall and sedimentation effects. The proposed methodology is tested on polymer blends of polybutene and silicon oil, and a good reproducibility is found.