We show how the surface coverage of partially n-octadecyltrichlorosilane (OTS) grafted silicon wafer can be estimated from dynamical wetting measurements. By use of the molecular kinetic theory of wetting, values of the effective jump frequency are determined from the relaxation of the contact angle of branched alkane droplets on such heterogeneous substrates. These jump frequencies are correlated to the OTS thickness measured by ellipsometry. The surface composition can then be calculated, indicating the strong influence of the transition from the expanded to the condensed liquid phase on the wetting properties of the substrate.