  The presence of fluctuating local relaxation times, $\tau_r(t)$ has been used
for some time as a conceptual tool to describe dynamical heterogeneities in
glass-forming systems. However, until now no general method is known to extract
the full space and time dependent $\tau_r(t)$ from experimental or numerical
data. Here we report on a new method for determining the local phase field,
$\phi_r(t) = \int^{t} dt'/\tau_r(t')$ from snapshots $\{r(t_i)\}_{i=1...M}$ of
the positions of the particles in a system, and we apply it to extract
$\phi_r(t)$ and $\tau_r(t)$ from numerical simulations. By studying how the
phase field depends on the number of snapshots, we find that it is a well
defined quantity. By studying fluctuations of the phase field, we find that
they describe heterogeneities well at long distance scales.
