  Distinguishing the photon output of an accreting supermassive black hole
binary system from that of a single supermassive black hole accreting at the
same rate is intrinsically difficult because the majority of the light emerges
from near the ISCOs of the black holes. However, there are two possible signals
that can distinctively mark binaries, both arising from the gap formed in
circumbinary accretion flows inside approximately twice the binary separation.
One of these is a "notch" cut into the thermal spectra of these systems in the
IR/optical/UV, the other a periodically-varying excess hard X-ray luminosity
whose period is of order the binary orbital period. Using data from detailed
galaxy evolution simulations, we estimate the distribution function in mass,
mass ratio, and accretion rate for accreting supermassive black hole binaries
as a function of redshift and then transform this distribution function into
predicted source counts for these two potential signals. At flux levels
>~10^{-13}~erg~cm^{-2}~s^{-1}, there may be ~O(10^2) such systems in the sky,
mostly in the redshift range ~0.5 < z < ~1. Roughly 10% should have periods
short enough (<~5~yr) to detect the X-ray modulation; this is also the period
range accessible to PTA observations.
