  The distance of the Fermi-detected blazar gamma-ray emission site from the
supermassive black hole is a matter of active debate. Here we present a method
for testing if the GeV emission of powerful blazars is produced within the
sub-pc scale broad line region (BLR) or farther out in the pc-scale molecular
torus (MT) environment. If the GeV emission takes place within the BLR, the
inverse Compton (IC) scattering of the BLR ultraviolet (UV) seed photons that
produces the gamma-rays takes place at the onset of the Klein-Nishina regime.
This causes the electron cooling time to become practically energy independent
and the variation of the gamma-ray emission to be almost achromatic. If on the
other hand the gamma-ray emission is produced farther out in the pc-scale MT,
the IC scattering of the infrared (IR) MT seed photons that produces the
gamma-rays takes place in the Thomson regime, resulting to energy-dependent
electron cooling times, manifested as faster cooling times for higher Fermi
energies. We demonstrate these characteristics and discuss the applicability
and limitations of our method.
