SEDs
immediately show AGNs dont look like stars
Multi-wavelength
works (e.g., radio, X-ray, IR, plus optical)
E.g.,
FIRST Bright Quasar Survey
Also
possible to find via
Variability
(e.g., MACHO)
Proper
Motion (or lack thereof, unbiased but tough!)
Grism
Surveys (e.g., Large Bright Quasar Survey)