Join Date: Feb 2009
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It is the brake PADS, not the rotors, that typically include a "wear" indicator. This is a metal strip/bracket attached to the pad commonly referred to, in slang terminology, as the "squealer."
When the pad wears to a predetermined thickness, the "squealer" contacts the surface of the rotor, producing a squealing, scraping, or other noise. The noise is indicative that you should replace the brake pads.
At the same time, you should also have the rotors examined. If they are excessively worn, as in below their "minimum thickness," then they should be replaced. A trained mechanic will typically use a micrometer to measure the rotor's thickness and compare it with factory specifications.
Rusting of rotors (and drums, for that matter) is contingent upon their (metal) composition, humidity, and air temperature. It is also contingent upon how long you may allow your car to sit before you drive it again. Most of the time, it's not anything to worry about. Example given: I can drive my '02 today, in a rain storm. Tomorrow, I will typically find light, surface oxidation (rust) on all four rotors. (My car has four-wheel disc brakes.) No big deal, either. As soon as I go down the road, the pads will clear it off.
All the above said, the brakes ARE a very critical safety component/system, and should be checked on a regular basis by a qualified mechanic. I have mine checked during every oil change.