The main way is through the presence of active galactic nuclei (AGN). If the black hole accretes gas rapidly enough, it can become very luminous, sometimes bri

The main way is through the presence of active galactic nuclei (AGN). If the black hole accretes gas rapidly enough, it can become very luminous, sometimes brighter than all the stars of the galaxy combined. One thing astronomers have looked for has been two AGN in the center of a galaxy (known as dual AGN). I think the main way that they are detected is in the X-ray since the two AGN generally have different spectra, so it's possible to distinguish them. (I'm not an X-ray astronomer, though, so I may be way off base.) But these searches for dual AGN have found that they are very rare relative to single AGN.