  The fundamental parameters describing the coronal spectrum of an accreting
black hole are the slope $\Gamma$ of the power-law continuum and the energy
$E_{cut}$ at which it rolls over. Remarkably, this parameter can be accurately
measured for values as high as 1 MeV by modeling the spectrum of X-rays
reflected from a black hole accretion disk at energies below 100 keV. This is
possible because the details in the reflection spectrum, rich in fluorescent
lines and other atomic features, are very sensitive to the spectral shape of
the hardest coronal radiation illuminating the disk. We show that fitting
simultaneous NuSTAR (3-79 keV) and low-energy (e.g., Suzaku) data with the most
recent version of our reflection model RELXILL, one can obtain reasonable
constraints on $E_{cut}$ at energies from tens of keV up to 1 MeV, for a source
as faint as 1 mCrab in a 100 ks observation.
