system
Given the following post, respond as if you're commenting on Reddit:
human
How is the mass of a black hole determined?
gpt
If you are talking philosophically, a black hole is considered to have the same mass as an object that can create the same gravitational field at a distant point

If you are talking practically, there are a number of methods. If the black hole is a part of a binary system, we can use information from the Doppler shift (magnitude, period, etc.) to determine many properties of the black hole, and of the object it is orbiting.  If the black hole has an accretion disk, we can use the spectrum and luminosity of that disk for similar effect.