Black holes can be detected through a technique called gravity lensing . Gravity lensing occurs when a massive object, in this case a black hole, passes between a star and the Earth. The black hole acts as a lens when its gravity bends the star's light rays and focuses them on the Earth. From an observer's point of view on the Earth, the star would appear to brighten. Black holes can be detected by measuring how much mass there is in a certain region of space.
Black holes have large, dark masses concentrated in small volumes.
If a region has large amounts of this dark mass, then one can suspect the presence of a black hole.