  We calculate the observable signature of a black hole accretion disk with a
gap or hole created by a secondary black hole embedded in the disk. We find
that for an interesting range of parameters of black hole masses (~10^6 to 10^9
solar masses), orbital separation (~1 AU to ~0.1 pc), and gap width (10 to 190
disk scale heights), the missing thermal emission from a gap manifests itself
in an observable decrement in the spectral energy distribution. We present
observational diagnostics in terms of power-law forms that can be fit to
line-free regions in AGN spectra or in fluxes from sequences of broad filters.
Most interestingly, the change in slope in the broken power-law is almost
entirely dependent on the width of gap in the accretion disk, which in turn is
uniquely determined by mass ratio of the black holes, such that it scales
roughly as q^(5/12). Thus one can use spectral observations of the continuum of
bright active galactic nuclei to infer not only the presence of a closely
separated black hole binary but also the mass ratio. When the black hole merger
opens an entire hole (or cavity) in the inner disk, the broad band SED of the
AGN or quasar may serve as a diagnostic. Such sources should be especially
luminous in optical bands but intrinsically faint in X-rays (i.e., not merely
obscured). We briefly note that viable candidates may have already been
identified, though extant detailed modeling of those with high quality data
have not yet revealed an inner cavity.
