  New heavy neutral gauge bosons Z' are predicted by many models of physics
beyond the Standard Model. It is quite possible that Z's are heavy enough to
lie beyond the discovery reach of the CERN Large Hadron Collider LHC, in which
case only indirect signatures of Z' exchanges may emerge at future colliders,
through deviations of the measured cross sections from the Standard Model
predictions. We discuss in this context the foreseeable sensitivity to Z's of
W^\pm-pair production cross sections at the e^+e^- International Linear
Collider (ILC), especially as regards the potential of distinguishing
observable effects of the Z' from analogous ones due to competitor models with
anomalous trilinear gauge couplings (AGC) that can lead to the same or similar
new physics experimental signatures at the ILC. The sensitivity of the ILC for
probing the Z-Z' mixing and its capability to distinguish these two new physics
scenarios is substantially enhanced when the polarization of the initial beams
and the produced W^\pm bosons are considered. A model independent analysis of
the Z' effects in the process e^+e^- \to W^+W^- allows to differentiate the
full class of vector Z' models from those with anomalous trilinear gauge
couplings, with one notable exception: the sequential SM (SSM)-like models can
in this process not be distinguished from anomalous gauge couplings. Results of
model dependent analysis of a specific Z' are expressed in terms of discovery
and identification reaches on the Z-Z' mixing angle and the Z' mass.
