  The insulating state of matter can be probed by means of a ground state
geometrical marker, which is closely related to the modern theory of
polarization (based on a Berry phase). In the present work we show that this
marker can be applied to determine the metal-insulator transition in disordered
systems. In particular, for non-interacting systems the geometrical marker can
be obtained from the configurational average of the norm-squared one-body
density matrix, which can be calculated within open as well as periodic
boundary conditions. This is in sharp contrast to a classification based on the
static conductivity, which is only sensible within periodic boundary
conditions. We exemplify the method by considering a simple lattice model,
known to have a metal-insulator transition as a function of the disorder
strength and demonstrate that the transition point can be obtained accurately
from the one-body density matrix. The approach has a general {\it ab-initio}
formulation and can be applied to realistic disordered materials by standard
electronic structure methods.
