A method to determine the surface azimuthal anchoring energy of a nematic liquid crystal is proposed. The technique implies the measurement of the director deviation on the cell substrate as a function of strength and direction of the applied magnetic field. As an example, the dependence of the azimuthal anchoring coefficient on the exposure time is measured at the interface between the nematic K 15 and polyvinylcinnamate film exposed by UV light. The analogous measurements performed in a wedge cell show that the method with magnetic field is more precise.