  The Novikov-Thorne model is the standard framework for the description of
geometrically thin and optically thick accretion disks around black holes and
is widely used to study the electromagnetic spectra of accreting black holes.
One of the assumptions of the model is that the particles of the gas move on
nearly-geodesic circular orbits on the equatorial plane. In this work, we
propose to test the Keplerian velocity of the particles in the accretion disk
using X-ray reflection spectroscopy. We present a modified version of RELXILL
in which we introduce a phenomenological parameter, $\alpha$, to quantify
possible deviations from Keplerian motion. We use our model to fit a Suzaku
observation of the black hole binary GRS 1915+105. We find that the estimate of
$\alpha$ is correlated to that of the inclination angle of the disk, $i$, and
that we could test the Keplerian disk hypothesis in the presence of a robust
and independent measurement of $i$.
