  If intermediate mass black holes (IMBHs) grow efficiently in gas disks around
supermassive black holes, their host active galactic nucleus (AGN) disks should
exhibit myriad observational signatures. Gap-opening IMBHs in AGN disks can
exhibit spectral features and variability analagous to gapped protoplanetary
disks. A gap-opening IMBH in the innermost disk imprints ripples and
oscillations on the broad Fe K$\alpha$ line which may be detectable with future
X-ray missions. A non-gap-opening IMBH will accrete and produce a soft X-ray
excess relative to continuum emission. An IMBH on a retrograde orbit in an AGN
disk will not open a gap and will generate soft X-rays from a bow-shock
'headwind'. Accreting IMBH in a large cavity can generate ULX-like X-ray
luminosities and LINER-like optical line ratios from local ionized gas. We
propose that many LINERs house a weakly accreting MBH binary in a large central
disk cavity and will be luminous sources of gravitational waves (GW). IMBHs in
galactic nuclei may also be detected via intermittent observational signatures
including: UV/X-ray flares due to tidal disruption events, asymmetric X-ray
intensity distributions as revealed by AGN transits, quasi-periodic
oscillations and underluminous Type Ia supernovae. GW emitted during IMBH
inspiral and collisions may be detected with eLISA and LIGO, particularly from
LINERs. We summarize observational signatures and compare to current data where
possible or suggest future observations.
