  We study how observationally to distinguish between a cosmological constant
(CC) and an evolving dark energy with equation of state $\omega(Z)$. We focus
on the value of redshift Z* at which the cosmic late time acceleration begins
and $\ddot{a}(Z^{*}) = 0$. Four $\omega(Z)$ are studied, including the
well-known CPL model and a new model that has advantages when describing the
entire expansion era. If dark energy is represented by a CC model with $\omega
\equiv -1$, the present ranges for $\Omega_{\Lambda}(t_0)$ and $\Omega_m(t_0)$
imply that Z* = 0.743 with 4% error. We discuss the possible implications of a
model independent measurement of Z* with better accuracy.
