  Notwithstanding the advent of the Gamma-ray Large Area Telescope, theoretical
models predict that a significant fraction of the cosmic gamma-ray background
(CGB), at the level of 20% of the currently measured value, will remain
unresolved. The angular power spectrum of intensity fluctuations of the CGB
contains information on its origin. We show that probing the latter from a few
tens of arcmin to several degree scales, together with complementary GLAST
observations of gamma-ray emission from galaxy clusters and the blazars
luminosity function, can discriminate between a background that originates from
unresolved blazars or cosmic rays accelerated at structure formation shocks.
