Differentiating various extra $Z^{\prime}$'s at Future Colliders
  We propose a way to differentiate between various extra $U(1)$ models using flavor tagging in the decay modes $Z^{\prime}\rightarrow q{\bar{q}}$ once the extra $Z^{\prime}$ is observed at future colliders in the lepton channel. A generalization of the R parameter, namely, one for charge $\frac{1}{3}$ and one for charge $\frac{2}{3}$ quark gives a two parameter test for the various models. Flavor tagging eliminates the uncertainty because of extra fermions and can reduce the QCD background at SSC/LHC dramatically. For $E_{6}$ and $SO(10)$ based models the former is always 3. This seems to be a very good way to eliminate certain models. 
P. K. Mohapatra Pages 9 COLO-HEP-271