  It is often stated that if one is presented with a snapshot of the positions
of the molecules of a glass and one of a liquid, one is unable to tell the
difference. Here we argue instead that given several such snapshots taken over
a time-interval, even without specifying the times, there is a definite
procedure to assess precisely the level of glassiness: it suffices to
concatenate the snapshots side-by-side, and to subject the joint picture to a
lossless compression protocol. We argue that the size of the compressed file
yields a direct and unambiguous measure of the `vibrational' and
`configurational' entropies, and may be used to study the associated glass
length scale in or out of equilibrium through the size and frequency of the
repeated motifs essential to the compression, a quantity that would diverge at
a putative glass transition.
