Inferring black hole charge from backscattered electromagnetic radiation
  We compute the scattering cross section of Reissner-Nordstr\"om black holes for the case of an incident electromagnetic wave. We describe how scattering is affected by both the conversion of electromagnetic to gravitational radiation, and the parity-dependence of phase shifts induced by the black hole charge. The latter effect creates a helicity-reversed scattering amplitude that is non-zero in the backward direction. We show that from the character of the electromagnetic wave scattered in the backward direction it is possible, in principle, to infer if a static black hole is charged. 
Lu\'is C. B. Crispino, Sam R. Dolan, Atsushi Higuchi and Ednilton S.   de Oliveira
gr-qc