There are various theoretical models for accretion disks and relativistic jets from black holes that considers the black hole to be either rotating (Kerr geometry) or non-rotating (Schwarzschild geometry). But when astronomers detect a source, they detects whether the source is rotating or non-rotating. One such example is the GRS 1915+105 which is an X-ray binary star system featuring a regular star and a black hole is observed to rotate at close to 1,150 times per second with a spin parameter value between 0.82 and 1.00 (maximum possible value).
So my question is the following:
How do astronomers detect whether a black hole is rotating or non-rotating? And if the black hole is rotating, how do they calculate the spinning rate?