re "tangible" -- Not sure what Mike means above, but besides the visual inspection, I always run a thumbnail over the rotor to see if I feel anything -- anything other than the normal rounded circumferential grooves.

I check the runout with a dial indicator too - mainly just for the hell of it. Like he & others have said, if it's your car, you'll feel it if things are wrong.

The exceptions would be cracking (esp. around drilled holes) which can quickly spread, and usage of the brakes in a manner they are not normally used in -- say a high speed or long mtn. downhill panic stop, where you usually don't test them that way.

I just had mine turned for $25 a pair. They measured 23.5 mm and had some slight grooving, due to a stuck piston and the original pads - one had 7 mm wear left, the other was less than 1 mm, down to the sensor, which I will delete on all four wheels. I'm rebuilding the calipers, too. I initially broke the glaze with a pad sander and some 100 grit, but they didn't feel smooth and flat, a little ridgy. The guy who did it took them down slightly on the disc lathe then cross-hatched the surface with some kind of abrasive wheel. They look really good now and measure just over 23 mm. I figured do it once and be done with it.
The Porsche rotors are much more rugged than any other rotors I've worked with, so I can't imagine warpage is a huge problem.

Regards,