  We propose an experimental technique for classifying the topology of band
structures realized in optical lattices, based on a generalization of
topological charge pumping in quantum Hall systems to cold atom in optical
lattices. Time-of-flight measurement along one spatial direction combined with
in situ detection along the transverse direction provide a direct measure of
the system's Chern number, as we illustrate by calculations for the Hofstadter
lattice. Based on an analogy with Wannier functions techniques of topological
band theory, the method is very general and also allows the measurement of
other topological invariants, such as the $Z_2$ topological invariant of
time-reversal symmetric insulators.
