Black Holes do not emit any light, so we cannot observe them directly. However objects falling into Black Holes emit X-Rays, so we can observe those emitted X-Rays and then deduce about the invisible Black Holes.

Alternatively, we can observe a shining star orbiting an invisible central object; and from the speed and distance from the invisible central object we can estimate the mass of the central object. Knowing the mass and volume of the central object we can decide whether it is a Black Hole candidate or not.