  In most of the models beyond the Standard Model, the top quark is expected to
be polarized when produced in the decay of some heavier particle, like the
gluino or the stop. The polarization is constructed, in an experiment or in
simulations, through the distribution of top decay products. Here, we propose
an estimator of top quark polarization that depends only on the kinematics of
it's mother particle, apart from its decay couplings to top quarks, and is
given in terms of the top polarization expected in the rest frame of the
decaying particle. This estimator allows one to estimate the top polarization
without performing a full simulation. We find this estimator is independent of
the production angle of the mother, top decay angle (for unpolarized mother),
and the spin of the mother particle. We study the quality of the estimator with
finite width of the mother particle via examples of gluinos and stops decaying
into top quark at LHC. We also point out how for the mass spectra of gluinos
and top squarks currently expected in a `natural' scenario, the polarization of
the top quarks produced in the gluino decays can uniquely track the mixing
angle in the stop sector.
