When you look up at space through a telescope, if you can find a blankness in space -such as a white piece of paper with a black hole in the middle and nothing around it, that could be one. They also found them by what they are sucking in. Planets, superheated gasses, more.
They know because the effect the black hole has on other matter.
No the singularity is at the core of the black hole.
Don't know which black hole.
Because there is usually nothing around the spot where the black hole is because the black hole "ate" everything.
We know nothing about the conditions within a black hole, but it seems unlikely that a black hole could exist within a black hole, or even if this concept would have any meaning at all.
Astronomers know because they have studied star patterns and placements and know when there has been a change between where there is a black hole and where there isn't one.
As far as we know, most, if not all galaxies have a massive black hole at their centre.
people now know more about the black hole.
no the galaxy is way to big for a black hole to do much in fact we now know that there is a supermassive black hole in the center of our galaxy right now.
None that we know of.
No one we know about
It is impossible to know what is present inside a black hole, though it is theorized that a singularity is located at its centre.