To distinguish between them, we need a spectrum. Astronomers were baffled. The bafflement was ended by the astronomer Maarten Schmidt in 1963. Schmidt discovered that the brightest emission lines in the spectra of quasars corresponded to the emission lines of ordinary atomic hydrogen, but that they were extremely redshifted . The bright quasar 3C 273 is actually one of the closest quasars to us.
Some quasars are much more distant.
The Hubble Law then states that these most distant quasars are approximately 4000 Mpc, or 14 billion light years, away. Thus, even the dimmest of the quasars are as bright as the brightest Seyfert galaxies and radio galaxies in the vicinity of the Milky Way. The luminosity of quasars can vary significantly on timescales of only a week. Quasars are now known to be very luminous and very distant active galaxies. The bulk of the luminosity comes from a region whose size is about ten times the Schwarzschild radius of the central black hole.