The inside of the electrical double layer at perovskite oxide heterointerfaces is examined. Here, we report the local polarization and valence distribution in LaNiO3/LaMnO3 and LaMnO3/LaNiO3 bilayers on a SrTiO3 (001) substrate. Simultaneous measurements of two aspects of the structure are realized by using Bayesian inference based on resonant- and nonresonant-surface x-ray diffraction data. The results show that the average Mn valences are Mn3.12+ and Mn3.19+ for the two samples. The intensity of their local electric field is ∼1 GV/m and the direction of the field points from LaMnO3 to LaNiO3.