  We discuss several possible experimental signatures of the Bose-Einstein
condensation (BEC) transition for an ultracold Bose gas in an inhomogeneous
optical lattice. Based on the commonly used time-of-flight imaging technique,
we show that the momentum-space density profile in the first Brillouin zone,
supplemented by the visibility of interference patterns, provides valuable
information about the system. In particular, by crossing the BEC transition
temperature, the appearance of a clear bimodal structure sets a qualitative and
universal signature of this phase transition. Furthermore, the momentum
distribution can also be applied to extract the condensate fraction, which may
serve as a promising thermometer in such a system.
