Invariant characterization of the Kerr spacetime: Locating the horizon   and measuring the mass and spin of rotating black holes using curvature   invariants
  We provide an invariant characterization of the physical properties of the Kerr spacetime. We introduce two dimensionless invariants, constructed out of some known curvature invariants, that act as detectors for the event horizon and ergosurface of the Kerr black hole. We also show that the mass and angular momentum can be extracted from local measurements of the curvature invariants, which in the weak field limit could be used to approximate the total angular momentum and mass of a system of merging black holes. Finally, we introduce a dimensionless invariant that gives a local measure of the "Kerrness" of the spacetime. 
Majd Abdelqader, Kayll Lake
gr-qc astro-ph.HE hep-th