1) Detection by Gravitational Lensing:
One way for researchers to detect a black hole is by its effect on the luminous matter around it. A black holes gravity is so strong that light passing by it will appear to be warped. This is the method known as gravitational lensing.
2) Detecting a Black Hole in a Binary Star System:
A binary system of stars that emits a large amount of X-rays is a likely place you will be able to find a black hole. The cause of these X-rays is from gas that has been taken from its companion star spiraling into the event horizon. The gas is heated by the tidal forces and friction from rubbing together with other gases falling into the black hole. This gas emits X-rays as it spirals into the event horizon. The combination of these forces causes the gas surrounding the black hole to be very visible until it actually falls into the event horizon making it easier for observers to spot them in a binary star system.
