  Verifying $V_{tb} \simeq 1$ is critical to test the three generation
assumption of the Standard Model. So far our best knowledge of $V_{tb}$ is
inferred either from the $3\times 3$ unitarity of CKM matrix or from single
top-quark productions upon the assumption of universal weak couplings. The
unitarity could be relaxed in new physics models with extra heavy quarks and
the universality of weak couplings could also be broken if the $Wtb$ coupling
is modified in new physics models. In this work we propose to measure $V_{tb}$
in the process of $e^+ e^- \to t\bar{t}$ without prior knowledge of the number
of fermion generations or the strength of the $Wtb$ coupling. Using an
effective Lagrangian approach, we perform a model-independent analysis of the
interactions among electroweak gauge bosons and the third generation quarks,
i.e. the $Wtb$, $Zt\bar{t}$ and $Zb\bar{b}$ couplings. The electroweak symmetry
of the Standard Model specifies a pattern of deviations of the $Z$-$t_L$-$t_L$
and $W$-$t_L$-$b_L$ couplings after one imposes the known experimental
constraint on the $Z$-$b_L$-$b_L$ coupling. We demonstrate that, making use of
the predicted pattern and the accurate measurements of top-quark mass and width
from the energy threshold scan experiments, one can determine $V_{tb}$ from the
cross section and the forward-backward asymmetry of top-quark pair production
at an {\it unpolarized} electron-positron collider.
