Membrane fouling may be dissected into four types: complete pore blocking, incomplete pore blocking, standard pore blocking and external surface fouling (cake/gel deposition). Using our experimental data on nanofiltration (NF) of NaCl solutions containing biosodiummethylene-bis-naphthalene sulphonate (BSMNS) as a foulant, we have tested some theoretical approaches with these data to recognize the type of membrane fouling. For fouling identification the following tools were used: (1) the diagrammatic representation of the fouling kinetics in the characteristic coordinates for constant pressure dead-end filtration; (2) the fitting of nux-time theoretical relationships recently modified for crossflow filtration to the experimental data; (3) the flux-pressure diagrams. The applicability of both the dead-end and crossflow filtration relationships to recognize the types of fouling at the initial stage of NF of the solutions studied has been shown. For a dilute BSMNS solution with a low salt concentration, pore blocking has been revealed as the contributory form of fouling. For the solution with a high salt concentration, the cake/gel build-up is the dominant mechanism of fouling. The discovered forms of fouling have been confirmed by the profile of the flux-pressure diagrams.