  Context. Binary systems of supermassive black holes are expected to be strong
sources of long gravitational waves prior to merging. These systems are good
candidates to be observed with forthcoming space-borne detectors. Only a few of
these systems, however, have been firmly identified to date.
  Aims. We aim at providing a criterion for the identification of some
supermassive black hole binaries based on the characteristics of the
high-energy emission of a putative relativistic jet launched from the most
massive of the two black holes.
  Methods. We study supermassive black hole binaries where the less massive
black hole has carved an annular gap in the circumbinary disk, but nevertheless
there is a steady mass flow across its orbit. Such a perturbed disk is hotter
and more luminous than a standard thin disk in some regions. Assuming that the
jet contains relativistic electrons, we calculate its broadband spectral energy
distribution focusing on the inverse Compton up-scattering of the disk photons.
We also compute the opacity to the gamma rays produced in the jet by photon
annihilation with the disk radiation and take into account the effects of the
anisotropy of the target photon field as seen from the jet.
  Results. We find that the excess of low-energy photons radiated by the
perturbed disk causes an increment in the external Compton emission from the
jet in the X-ray band, and a deep absorption feature at energies of tens of
TeVs for some sets of parameters. According to our results, observations with
Cherenkov telescopes might help in the identification of supermassive black
hole binaries, especially those black hole binaries that host primaries from
tens to hundreds of million of solar masses.
