You didn't say what kind of rf camera you're looking at and what kind of condition you're willing to accept (mint? knocked around but good user? fixer upper?).
The answer varies depending if you're looking at used Leicas, VC Bessas, Russian cameras, or (relatively) cheap and cheerful 70's fixed lens rfs.
And if interchangeable lens rf, what kind of lenses.. (M mount, screwmount, contax mount), same brand as the camera, 3rd party German-Japanese or Russian?
Basically, if you're looking at a fairly new, clean, seemingly functional (shutter and focusing works) camera then put film through it for test shots.
-Run through the range of shutter speeds and apertures to test for consistent exposure.
-Check for horizontal and vertical rf patch alignment. Misalignment on Leicas is serious and expensive to fix, very minor if on VC Bessas, and not worth paying to fix on cheap and cheerful rfs.
-Focus on near and distant objects. Look at the printed pix to verify focus accuracy.
-Check the negs for overlapping frames. Intermittent uneven spacing between frames is still acceptable. Again, this could be bad news if on a Leica. Acceptable and probably normal on some VC Bessas - depending how hard or fast the film advance lever was cranked.
-Inspect the shutter when cocked and uncocked. If cloth (or rubberized cloth), the curtains should not have wrinkles or pinholes. On metal shutters, check for bent blades. Shutter blades on well used cameras might show light scuff marks but that should be ok.
Btw, Russian rf lenses are low cost (but not dirt cheap anymore) and good but best used on Russian camera bodies. They tend to focus differently on other bodies because of design differences.
Hope this helps.
