  It is known that for an unconventional superconductor with nodes in the gap,
the in-plane microwave or dc conductivity saturates at low temperatures to a
universal value independent of the impurity concentration. We demonstrate that
a similar feature can be accessed using channel-dependent Raman scattering. It
is found that, for a $d_{x^2-y^2}$-wave superconductor, the slope of
low-temperature Raman intensity at zero frequency is universal in the $A_{1g}$
and $B_{2g}$ channels, but not in the $B_{1g}$ channel. Moreover, as opposed to
the microwave conductivity, universal Raman slopes are sensitive not only to
the existence of a node, but also to different pairing states and should allow
one to distinguish between such pairing states.
