The ringdown of a black hole formed in a binary merger is conventionally treated as a sum of quasinormal modes, fixed entirely by the remnant's mass and spin. This description presumes the spacetime is already close to the idealized, stationary Kerr solution. This talk asks what it means for a dynamical merger remnant to be "close to Kerr", and presents a framework for measuring how close it actually is. Building on necessary and sufficient conditions for a spacetime to be isometric to Kerr, encoded through the Petrov classification, principal null directions, and Killing symmetry, I track these conditions directly on a numerically evolved binary black hole spacetime and connect their decay near the black hole causally to the gravitational wave signal extracted at future null infinity. This provides a direct, first principles way to assess when a perturbative, quasinormal mode description of the remnant is justified, with immediate consequences for black hole spectroscopy and other gravitational wave tests of general relativity that rely on this assumption.