1980 308 rotors

It should be marked/stamped on the rotors themselves. The marking on mine was located on the edge of the hat right next the surface that the wheel bolts to. You probably need to take a wire brush and clean the metal a little bit to uncover it.

I did just resurface mine, but I don't remember the figure for sure, but I think the fronts were .707". But it doesn't matter because any good machine shop will find the marking and not turn the rotor if it is too thin.

 

I just happened to have the rear wheel off last night and the thickness stamping was staring right at me, so I took a couple of pictures. Here you go, one of the stamping and one of the general location of the stamping:
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The number (and picture) that Brian posted are for the (stock) rear 308 rotors -- the (stock) front 308 rotors have a larger minimum thickness:

front = 22 mm nominal, 20 mm minimum

rear = 20 mm nominal, 18 mm minimum