Constraining the nature of the Galactic center black hole Sgr A* with   present and future observations
  The Galactic center is an ideal laboratory to study strong-field general relativistic phenomena, as the supermassive black hole Sgr A* has the biggest angular Schwarzschild radius among all black holes. This article presents three different ways of using the immediate surroundings of Sgr A* as a probe in order to either constrain its spin, or even test the very nature of this compact object. 
F. H. Vincent, E. Gourgoulhon, O. Straub, M. Abramowicz, J. Novak, T.   Paumard and G. Perrin
astro-ph.HE gr-qc