  Weyl semimetals are a three dimensional gapless topological phase in which
bands intersect at arbitrary points -- the Weyl nodes -- in the Brillouin zone.
These points carry a topological quantum number known as the \emph{chirality}
and always appear in pairs of opposite chiralities. The notion of chirality
leads to anomalous non-conservation of chiral charge, known as the \emph{chiral
anomaly}, according to which charge can be pumped between Weyl nodes of
opposite chiralities by an electromagnetic field with non-zero
$\boldsymbol{E}\cdot\boldsymbol{B}$. Here, we propose probing the chiral
anomaly by measuring the optical activity of Weyl semimetals via circular
dichroism. In particular, we observe that applying such an electromagnetic
field on this state gives it a non-zero gyrotropic coefficient or a Hall-like
conductivity, which may be detectable by routine circular dichroism
experiments. This method also serves as a diagnostic tool to discriminate
between Weyl and Dirac semimetals; the latter will give a null result. More
generally, any experiment that probes a bulk correlation function that has the
same symmetries as the gyrotropic coefficient can detect the chiral anomaly as
well as differentiate between Dirac and Weyl semimetals.
