Hello Mr. Alex. The most direct way to measure a skin layers (dense, non-porous portion) of a membrane is by direct observation with FESEM at high resolutions.
Alternatively, particularly for gas separation application, one can estimate the skin layer by backwards calculation from the equation to calculate gas permeance once the intrinsic permeability of the gas is determined from fully dense membrane. Example by Pesek and Koros (1994)