  We show how an observer could measure the non-local holonomy variables that
parametrise the flat Lorentzian 3d manifolds arising as spacetimes in
(2+1)-gravity. We consider an observer who emits lightrays that return to him
at a later time and performs several realistic measurements associated with
such returning lightrays: the eigentime elapsed between the emission of the
lightrays and their return, the directions into which the light is emitted and
from which it returns and the frequency shift between the emitted and returning
lightray. We show how the holonomy variables and hence the full geometry of
these manifolds can be reconstructed from these measurements in finite
eigentime.
