Question

If scientists can't actually see black holes how are they studied?

Answer 1

Since nothing can escape from Black holes we have to use indirect methods.

Explanation:

Stars orbiting black holes will have very high speed depend upon the mass of black hole.
In binary stars when one become black hole,material from the other will be attracted by black hole.They will produce X rays due to very high temperature at the event horizon.
Picture credit.Physics.org.
These indirect methods are proofs for black holes.