Blackhole Finding Techniques
We detect black holes mostly by their effects on nearby matter. The "radial velocity" method is important after x-ray observations suggest a likely black-hole.

Matter in orbit around a black hole can form an accretion disc. This disc will become very hot, and glow brightly in X-ray radiation. If we see X-ray radiation coming from a star we look at the radial velocity of the star, and use this to infer the mass of the orbiting object. Since a neutron star cannot exist at more than 3 solar masses, if the object is more than that it must be a black hole.

Compared to planets, black holes are rare. Only about 20 candidates are known in the Milky way.