  We study the diagnostic power of future $e^+e^-$ colliders with ${\sqrt {s}}
= 500$ GeV (the New Large Collider ($NLC$)) for a model independent
determination of the $Z'$ gauge couplings to quarks and leptons. The
interference of the $Z'$ propagator with the photon and the $Z$ propagator in
the two-fermion final state probes are sensitive to the magnitude as well as
relative signs of quark and lepton charges. For $Z'$ with $M_{Z'} \sim 1 $ TeV
{\it all } the quark and lepton charges can be determined to around $10-20 \%$,
provided heavy flavor tagging and longitudinal polarization of the electron
beam is available. The errors are $2-10$ times larger without polarization, and
very little information can be obtained about quark charges without heavy
flavor tagging. We point out the complementarity of future hadron colliders. At
the CERN Large Hadron Collider ($LHC$) primarily the magnitude of three out of
four corresponding couplings can be measured; however, their error-bars are
typically by a factor of $\sim 2$ smaller than those at the $NLC$.
