  Quasi-normal modes (QNMs) uniquely characterize the final black-hole. Till
now, only the QNM frequency and damping time are used to test General
relativity. In this work, we show explicitly that another property of the QNMs
--- their polarization --- can be a reliable tool for probing gravity. We
provide a consistent test for General relativity by considering Chern-Simons
gravity. Distinguishing Chern-Simons gravity from General relativity using only
template matching is highly challenging. Thus a parameter that can
differentiate between Chern-Simons gravity and GR will be a suitable candidate
for any modified theories of gravity. We discuss the implications of our result
for the future gravitational wave detectors.
