  Determining the spin of any new particle is critical in identifying the true
theory among various extensions of the Standard Model (SM). The degree of
$Z$-boson polarization in any two-body decay process $A\to B Z$ is sensitive to
the spin assignments of two new particles $A$ and $B$. Considering all possible
spin-0, 1/2 and 1 combinations in a renormalizable field theory, we demonstrate
that $Z$-boson polarization can indeed play a role of a decisive and universal
analyzer in distinguishing the different spin assignments.
