I often purchase older S&W revolvers, usually the N-Frame size. I look for hammer push-off, excessive end shake, appropriate/firm lock-up, ratchet teeth condition, hammer-nose condition, pitting, rust, bluing (unless stainless), overall condition, original stocks, and the condition of the action. The seller won't normally allow removal of the side plate, but if they do, look at the internals.
If the seller won't let me dry-fire the revolver, I won't purchase.