  Microwave conductivity experiments can directly measure the quasiparticle
scattering rate in the superconducting state. We show that this, combined with
knowledge of the Fermi surface geometry, allows one to distinguish between
closely related superconducting order parameters, e.g., d$_{x^2-y^2}$ and
d$_{xy}$ superconductivity. We benchmark this method on
YBa$_2$Cu$_3$O$_{7-\delta}$ and, unsurprisingly, confirm that this is a
d$_{x^2-y^2}$ superconductor. We then apply our method to
$\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br, which we discover is a d$_{xy}$
superconductor.
