On cars with the older style slave cylinder it is possible to estimate the amount of disk wear.  Remove the access cover for the slave bleed nipple.  See the black plastic cap?  See the rim around it (right side of picture below, or toward the top in this other picture)?  OK, that rim starts out 11 mm away from the front face of the slave cylinder.  You can see it better in this other picture.  When the rim on the plastic dust cover is 2 mm away from the front face of the slave cylinder, the disc is worn out.  It's real easy to see on a 900, but because of the way the 9000 is designed, it's not at all easy to see.  There is a screw that secures the slave to the trans, and it's about 3 mm thick.  If the plastic cap rim is behind the screw, then the clutch is pretty much shot.  You might be able to get some sort of cloth on a stick down in there to clean things off enough to be able to see.  As you can see, I still had about 6 mm of distance.  The disc wasn't shot but the release bearing was, and the clutch operation was pretty raggedy.