Answer:
The minimum thickness will be stamped somewhere. You see there's part numbers and other things here you might need to use a wire brush to kind of clean it up but that's the thicknessUse a micrometer at the thinnest point on the surface of the rotor where it meets the pads to measure the minimum operational thickness. If your rotor needs to be cut or has worn passed this thickness, you'll need new rotors. You may also discover your rotor has Disc Thickness Variation.