If you are running stock calipers, feel around the backside of the Rotor's edge. You should feel a significant lip there that indicated how far down your rotors are worn.
On Brembo's the front side wears about as fast as the back (Since there are pistons on both sides) So you can just run your finger long the top of th rotor.
Mine wore almost 1.5mm - 2mm in about 500miles. Not very good considering the fact that they were 28mm to begin with.
Worn rotors (not cracked or damaged) look about the same as normal Rotors except that there is a larger lip on the edge. I'd check the thickness with a caliper and see if they are within spec.
Brembo wants 398.00 PER Rotor for replacing those babies so make sure they last!!!
Btw if you go to Wilwood, they can custom fabricate the same Rotors for 125.00 for both I think...
Maybe it was 125.00 a peice...
In any case, the Wilwood Rotors worked alot better for me than the Brembo's.
My Brembo Rotors cracked and warped and just about fell apart after 2 track events.