A few things in addition to those mentioned (I use for all guns)
1) Stripped screws in wood or metal--stripped wood is an easy fix, metal screws--not so much. If anything is loose, bring a screwdriver and ask you can check. I've seen screw heads broken off from the shank and just hanging in there. This can get expensive if it's a scope mount screw that needs to have the old shank take out or be re-tapped.
3) Lever guns--check for lever droop--does the lever meet firmly at the tang, or does it droop so there is a gap? Again, some guns are easy to fix, some, not so much.
4) Shims that shouldn't be there
5) Holes that shouldn't be there in wood or metal.
6) Check for matching serial numbers on forearm, stock and metal--but ASK first (if this is even a concern for a shooter)
7) Buggered screw heads--might not tell you much but if Bubba worked on it on the back porch with a butter knife it might give you a heads up of other things look amiss on the gun.
8) Does the safety work?
9) Any welds or brazing on the lever or tangs, that might have been touched up with blue? We all get old and broken sometime, but still nice to know. A fix is a fix but if it was fixed well you might not even see it at first pass.

None of these would be deal killers, but just might help you make a better decision, or save you a few bucks.

Last edited by SteveS; 02/15/11.