Loose oxide issues

Hi all, A friend of mine is having some issues with old disks (Apple II, from the early 80s). He's finding that on trying to access them, the drive becomes unresponsive and won't read even disks previously known to be good. He says that they don't show signs of foreign matter/growth, so I'm guessing they're losing oxide onto the head. He's sent me his drives and I've sent him some of mine; his ones were fine after a head clean, but definitely nonfunctional before that. So, my question is: is there any way he can tell whether a particular disk has this loose oxide problem, without running the disk in a drive? The drives in question aren't very easy to open and clean manually (later Unidisk plastic-case drives), and he has no cleaning disks (though they are pretty rough on heads anyway, I hear). Perhaps gently rubbing the disk with a cotton bud might reveal very loose oxide without damaging the disk too much, but I doubt the disk would release enough to discolour the bud. I'm actually thinking about making a "safe" cleaning disk, perhaps with smooth plastic on the unused side, and some sot of low-abrasive cloth on the other. Even for me, that would be easier than pulling a drive to bits every time the head gets dirty. Thoughts, anyone? Thanks, Mike