Ferrite devices based on Faraday rotation remain of interest at both microwave and millimeter wavelengths. Although a complete mathematical description of ferrite cylinders in round waveguides is available, the experimental verification of rotation or differential phase shift in a round waveguide is not simple. The paper describes a resonant cavity procedure which permits the evaluation of the two propagation constants in such a magnetised waveguide. It is obtained by incorporating the Faraday rotator section into a resonator structure by closing both ends of the waveguide and separately tuning its length for each of the two split branches of the magnetised resonator to the frequency of the demagnetised one. The paper includes a lower bound on the length of the rotator bit (or an upper bound on the splitting between the phase constants) in terms of the directivity of the section.