  We propose an experiment to use the magneto-optical Faraday effect to probe
the dynamic Hall conductivity of spin liquid candidates. Theory predicts that
an external magnetic field will generate an internal gauge field. If the source
of conductivity is in spinons with a Fermi surface, a finite Faraday rotation
angle is expected. We predict the angle to scale as the square of the frequency
rather than display the standard cyclotron resonance pattern. Furthermore, the
Faraday effect should be able to distinguish the ground state of the spin
liquid, as we predict no rotation for massless Dirac spinons. We give a
semiquantitative estimate for the magnitude of the effect and find that it
should be experimentally feasible to detect in both
$\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ and, if the spinons form a Fermi surface,
Herbertsmithite. We also comment on the magneto-optical Kerr effect and show
that the imaginary part of the Kerr angle may be measurable.
