How can you tell if theres a black hole?

Question: How can you tell if theres a black hole?
- Keywords:
  black hole,
  detection,
  equipment
Asked by pandalover to Manolis on 10 Mar 2014. This question was also asked by .
- Manolis Rovilos answered on 10 Mar 2014:
  
  You cannot see a black hole directly, but you can see gas falling into it and being heated by its energy, or stars orbiting it.

Question: How can you tell if theres a black hole?