If I ever think about buying a used revolver (or a new one for that matter). I check the end shake with a feeler gauge and I will carry a range rod of that caliber with me and check every indexed position.
There are a lot of other things I check to, but those two are the first.
When I was in police academy, we were issued stainless S-R Security Sixes in .357. The staff were ‘gigging’ cadets for discoloration of the cylinder face, saying that it was indicative of lead build-up.. That would cost you your bi-weekly leave. (We forbidden to even leave the building without permission).
In response, the cadets got creative in removing said discoloration, which is a natural result of firing and not necessarily the result of lead build-up. Flitz and other polishes, wire brushes and emery paper entered the weapon maintenance toolkit. Soon, most of the revolvers had gaps between the cylinders and the forcing cones that you could slip a couple of playing cards through. Of course, the weapons were at that point ruined.
We did however get some awesome photos of shooters silhouetted by the huge circular blossom of fire emanating from their revolvers. Pretty.