A way to determine the magnetic axis of gyrotron cryomagnets, and more generally magnets composed of coils wound around a single mandrel is presented. The method is based on measurements of the longitudinal component of the magnetic field performed at suitable locations with a high accuracy (100 ppm) Hall probe [1]. The tilt and the shift of the magnetic axis with respect to the mechanical axis can be determined with a typical accuracy of 0.01deg and 0.1 mm respectively.