  We propose optical longitudinal conductivity as a realistic observable to
detect light-induced Floquet band gaps in graphene. These gaps manifest as
resonant features in the conductivity, when resolved with respect to the
probing frequency and the driving field strength. We demonstrate these features
via a dissipative master equation approach which gives access to a frequency-
and momentum-resolved electron distribution. This distribution follows the
light-induced Floquet-Bloch bands, resulting in a natural interpretation as
occupations of these bands. Furthermore, we show that there are population
inversions of the Floquet-Bloch bands at the band gaps for sufficiently strong
driving field strengths. This strongly reduces the conductivity at the
corresponding frequencies. Therefore our proposal puts forth not only an
unambiguous demonstration of light-induced Floquet-Bloch bands, which advances
the field of Floquet engineering in solids, but also points out the control of
transport properties via light, that derives from the electron distribution on
these bands.
