How Do We Know if a Black Hole Is Spinning?

We knew how massive it was, and that tells us how big it should be. The problem is, how do you actually measure its radius? How do you tell how big this thing is? All we had previously were stars that are whizzing around it, but they weren't coming back close to the black hole. They only came to within like 12 or 15 a u of this black hole. So we need to verify that it really was that compact, that it wasn't like a larger, fluffier object. It would probably amat our solar system collapse, right? Ecause it is four million times the mass of our sun, exactly. Not suggesting any bo y try