Fouling is generally described in terms of salt rejection and permeation flow, but these data seem to evolve very slowly, and when they reach a dramatic level fouling is irreversible. We propose a method to determine the state of the wear of membranes by analyzing sodium chloride stimulus-response experiments. Indeed, it turns out that the shape of the distribution (RTD) of sodium chloride in the permeate flow of the membrane reveals the solute permeation mechanisms for used membranes. For new membranes the distribution of sodium chloride collected in the permeate side as well in the rejection side is unimodal. For fouled membranes we note a singular distribution form with the presence of several modes. The existence of a salt leakage peak, as well as an earlier detection of salt for all the fouled membranes, give evidence of the membrane structure modification. The intensive use of the membranes might have created an enlargement of the pore sizes; salt and solvent permeabilities are improved as well. It turns out that each permeate side sodium chloride distribution can be fitted by a weighted average of Gaussian distributions, the new membrane RTD and the shifted new membrane RTD. The coefficients of this average give us the fraction of the membrane which is enlarged, hampered by the deposited layer and not modified.