The Faraday effect has ongoing applications including optical isolators and measuring dynamic magnetic fields in lab or space-based plasmas. A direct interferometric study of the circular birefringence underlying the Faraday effect provides a powerful platform for creative work in the undergraduate optics laboratory and makes clear the underlying physics. This experiment introduces a heterodyne interferometer for determining phase shifts between left- and righthanded circular polarizations as they pass through a substance in the direction of the magnetic field. Phase shifts due to magnetic fields within a commercially available 1.3 cm long terbium gallium garnet (TGG) crystal (after 3 passes of the 633 nm laser beam) can be plotted as a function of time – corresponding to B(t) down to the mT range. In the case of an broader advanced lab project format involving several weeks of time, the experiment may also provide a venue for introducing the use of heterodyne interferometers in measuring rapidly changing optical path length changes on the nanometer scale.