For MMMs, a common approach is to apply Maxwell-like theories, which treat the filler as a phase uniformly dispersed within the polymer matrix. These theories aim to provide an analytical expression for the effective permeability of a species within the composite, given the permeability values of each phase. Perm-selectivity is again calculated as the ratio of composite permeabilities for two species. These approaches, however, rely on certain assumptions. A primary challenge arises when theoretical predictions fail to align with experimental behavior. For example, for MMMs, the properties and behavior of interfaces between the filler and polymer are important in determining the overall MMM performance. Yet, the interfaces represent the most significant sources of uncertainty in theoretical models as it is impossible to accurately characterize these regions in experiments. Consequently, theoretical models must assign assumed values to the interfacial properties or fit them to the experimental data. As a result, theoretical studies are valuable for interpreting experimental data when they align; however, when they diverge from experimental results, it becomes difficult to determine whether the issue lies with the theory itself, technical limitations, or unaccounted factors within the experiments. Therefore, both theoretical methods and experimental studies need an independent, reference method to probe the assumptions within theoretical approaches and to diagnose the nature of the deviations of the properties of the membrane from the idealized expectations.