  The site and mechanism of gamma-ray burst (GRB) prompt emission is still
unknown. Although internal shocks have been widely discussed as the emission
site of GRBs, evidence supporting other emission sites have been also suggested
recently, including the closer-in photosphere where the fireball becomes
transparent and further-out radii near the fireball deceleration radius where
magnetic dissipation may be important. With the successful operation of the
GLAST experiment, prompt high energy emission spectra from many GRBs would be
detected in the near future. We suggest that the cut-off energy of the prompt
emission spectrum from a GRB depends on both the fireball bulk Lorentz factor
and the unknown emission radius from the central engine. If the bulk Lorentz
factor could be independently measured (e.g. from early afterglow
observations), the observed spectral cutoff energy can be used to diagnose the
emission site of gamma-rays. This would provide valuable information to
understand the physical origin of the GRB promp emission.
