need to check the length of disk

 

the minimum thickness will be stamped on the rotor, you need a caliper to measure it, you can get a cheap caliper from harbor freight, if you find the coupon should be about $10

https://www.harborfreight.com/6-inch...per-47257.html

or if you take it somewhere to get turned they will measure and tell you if it needs to be replaced

 

Measure? Take somewhere to be turned??

Rotors are like $10-20 each. Live your life.

 

Usually cracking, grooves.....signs that make the rotor look like there's something wrong is usually a good rule of thumb that they need to be replaced.

If your concerned and not sure just replace them they don't cost much. Better safe then sorry