Doppler Measurements
Direct Doppler measures of water masers surrounding the nuclei of nearby galaxies have revealed a very fast Keplerian motion, only possible with a high concentration of matter in the center. Currently, the only known objects that can pack enough matter in such a small space are black holes, or things that will evolve into black holes within astrophysically short timescales. For active galaxies farther away, the width of broad spectral lines can be used to probe the gas orbiting near the event horizon. The technique of reverberation mapping uses variability of these lines to measure the mass and perhaps the spin of the black hole that powers the active galaxy's "engine".
Such supermassive black holes in the center of many galaxies are thought to be the "engine" of active objects such as Seyfert galaxies and quasars.
Read more about this topic: Supermassive Black Hole