  Since it was theorized by Kerr in 1963, determining the spin of black holes
from observed data was paid very little attention until few years back. The
main reasons behind this were the unavailability of adequate data and the lack
of appropriate techniques. In this article, we explore determining/predicting
the spin of several black holes in X-ray binaries and in the center of
galaxies, using X-ray and gamma-ray satellite data. For X-ray binaries, in
order to explain observed quasi-periodic oscillations, our model predicts the
spin parameter of underlying black holes. On the other hand, the nature of spin
parameters of black holes in BL Lacs and Flat Spectrum Radio Quasars is
predicted by studying the total luminosities of systems based on Fermi
gamma-ray data. All sources considered here exhibit characteristics of spinning
black holes, which verifies natural existence of the Kerr metric.
