  We reconsider the effect of disorder on the properties of a superconductor
characterized by a sign-changing order parameter appropriate for Fe-based
materials. Within a simple two band model, we calculate simultaneously $T_c$,
the change in residual resistivity $\Delta \rho_0$, and the zero-energy density
of states, and show how these results change for various types of gap
structures and assumptions regarding the impurity scattering. The rate of $T_c$
suppression is shown to vary dramatically according to details of the impurity
model considered. We search therefore for a practical, experimentally oriented
signature of a gap of the $s_\pm$ type, and propose that observation of
particular evolution of the penetration depth, nuclear magnetic resonance
relaxation rate, or thermal conductivity temperature dependence with disorder
would suffice.
