Deciding whether a brake rotor can be reused comes down to one measurement: its thickness versus the minimum spec. A rotor thickness gauge (a micrometer) makes that a quick, precise check. Here is how to use one.
Why measure thickness
Rotors wear thinner over their life, and below a minimum thickness they overheat, warp, and can crack - see rotor minimum thickness. The only way to know if a rotor is safe to reuse or resurface is to measure it and compare to the discard number stamped on it - see where the minimum is stamped.
The right tool
Use a brake rotor micrometer - it has a pointed or ball anvil designed to reach past the raised outer lip and into the swept friction area. A flat caliper reads the lip, not the worn area, so it gives a falsely high number. The pointed micrometer measures the actual worn friction surface.
How to measure
Measure the rotor thickness at several points around the friction ring - typically 4 to 6 spots, about an inch in from the outer edge. Record the smallest reading. Measuring multiple spots also reveals thickness variation (a difference between points), which causes a pulsation - see brake judder. For the broader method see how to measure rotor thickness.
Reading the result
If the smallest reading is at or below the stamped minimum, replace the rotor - it cannot be safely reused or machined. If it is above minimum and even, the rotor is reusable. Replace rotors in pairs when at the limit, with quality pads and fresh hardware.