  An alternate method of measuring anisotropy fields in thin film ferromagnets
is demonstrated. The method relies on the magnetic susceptibility in a small
a.c. magnetic field, measured \txtit{in situ} using the magneto-optic Kerr
effect (MOKE), and will be useful in situations where more specialized
apparatus are not available, or constraints discourage the use of a large,
static magnetic field. The method is demonstrated for Co/W(110) films, where it
yields anisotropy fields in agreement with previous studies using more
conventional torque magnetometry. The sensitivity of the method is demonstrated
using CoO/Co/W(110) bilayer films, where the anisotropy due to interfacial
exchange coupling is detected and used to find the N\'{e}el temperature of the
thin CoO layer.
