  The super-massive objects in galactic nuclei are thought to be the Kerr black
holes predicted by General Relativity, although a definite proof of their
actual nature is still lacking. The most massive objects in AGN ($M \sim 10^9
M_\odot$) seem to have a high radiative efficiency ($\eta \sim 0.4$) and a
moderate mass accretion rate ($L_{\rm bol}/L_{\rm Edd} \sim 0.3$). The high
radiative efficiency could suggest they are very rapidly-rotating black holes.
The moderate luminosity could indicate that their accretion disk is
geometrically thin. If so, these objects could be excellent candidates to test
the Kerr black hole hypothesis. An accurate measurement of the radiative
efficiency of an individual AGN may probe the geometry of the space-time around
the black hole candidate with a precision comparable to the one achievable with
future space-based gravitational-wave detectors like LISA. A robust evidence of
the existence of a black hole candidate with $\eta > 0.32$ and accreting from a
thin disk may be interpreted as an indication of new physics. For the time
being, there are several issues to address before using AGN to test the Kerr
paradigm, but the approach seems to be promising and capable of providing
interesting results before the advent of gravitational wave astronomy.
