  In general relativity, the Kerr metric uniquely represents the geometry
surrounding an isolated, rotating black hole. An identification of significant
non-Kerr features in some astrophysical source would then provide a
`smoking-gun' for the break-down of general relativity in the strong-field
regime. On the other hand, Kerr black holes are common to many other theories
of gravity, and thus a validation of the Kerr metric does not necessarily
favour general relativity amongst all possibilities. The nature of
gravitational perturbations will however differ between different theories of
gravity. Future precision tests involving gravitational waves from oscillating
black holes, such as identifications of the quasi-normal mode spectrum from
ring-down, will thus be able to probe the underlying theory, even if the object
is Kerr. Here, we write down the equations governing metric perturbations of a
Kerr black hole in $f(R)$ gravity in a form that is more conducive to numerical
study.
