  An issue currently under debate in the literature is how far from the black
hole is the Fermi-observed GeV emission of powerful blazars emitted. Here we
present a clear diagnostic tool for testing whether the GeV emission site is
located within the sub-pc broad emission line (BLR) region or further out in
the few pc scale molecular torus (MT) environment. Within the BLR the
scattering takes place at the onset of the Klein-Nishina regime, causing the
electron cooling time to become almost energy independent and as a result, the
variation of high-energy emission is expected to be achromatic. Contrarily, if
the emission site is located outside the BLR, the expected GeV variability is
energy-dependent and with amplitude increasing with energy. We demonstrate this
using time-dependent numerical simulations of blazar variability.
