Ok, I've got a disc in my garage that I replaced after doing a trans swap and when I pulled it it mic'd the same thickness as the new spec unit I installed, it had very low milage.. So I went out and measured it and it is .300" thick. If you have a 0-1" mic or a dial caliper use that number to compare as new.
If you don't have a mic or caliper, you can use a scale(finely graduated ruler) with a good eye..
Personally, I wouldn't buy a used disc unless it was practically new. I don't know what the minimum usable thickness is, but hope this helps...