The inside of the electrical double layer at perovskite oxide heterointerfaces is examined. Here, we report the local polarization and valence distribution in LaNiO$_3$/LaMnO$_3$ and LaMnO$_3$/LaNiO$_3$ bilayers on a SrTiO$_3$ (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 Mn$^{3.12+}$ and Mn$^{3.19+}$ for the two samples. The intensity of their local electric field is $sim$1~GV/m and the direction of the field points from LaMnO$_3$ to LaNiO$_3$.
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