Wedge cells (of the wedge angle of the order of few milliradians) were used for studying Freedericksz' transition of splay-bend type. The phase shift between ordinary and extraordinary rays of light normally incident on the cell boundary was used as a physical quantity monitoring the state of the director field inside the cell in selected zones equivalent to flat-parallel cells of different thickness. The optical response was measured as a function of a voltage or a magnetic field (or both of them, crossed or parallel). Experiments were performed with two cells, filled with the 4'-pentyl-4-cyanobipfenyl liquid crystal (5CB), of different cover coatings providing strong or weak nematics-substrate coupling. Four different methods were applied for determining the anisotropy of diamagnetic susceptibility of the nematics after results of measurements, separately or simultaneously with the splay or bend elastic constants or the nematics-substrate coupling characteristics. The resulting values of material parameters are compared and the features of the methods are commented.