  We consider scalar perturbations of the Reissner--Nordstr\"{o}m family and
the Kerr family. We derive a characteristic expression of the radiation field,
at any given unit solid angle of future null infinity, and numerically show
that its amplitude gets excited only in the extremal case. Our work, therefore,
identifies an observational signature for extremal black holes. Moreover, we
show that the source of the excitation is the extremal horizon instability and
its magnitude is exactly equal to the conserved horizon charge.
