  We propose a nonparametric method to determine the sign of \gamma - the
redshift evolution index of dark energy. This is important for distinguishing
between positive energy models, a cosmological constant, and what is generally
called ghost models. Our method is based on geometrical properties and is more
tolerant to uncertainties of other cosmological parameters than fitting methods
in what concerns the sign of \gamma. The same parameterization can also be used
for determining $\gamma$ and its redshift dependence by fitting. We apply this
method to SNLS supernovae and to gold sample of re-analyzed supernovae data
from Riess et al. 2004. Both datasets show strong indication of a negative
\gamma. If this result is confirmed by more extended and precise data, many of
dark energy models, including simple cosmological constant, standard
quintessence models without interaction between quintessence scalar field(s)
and matter, and scaling models are ruled out.
