The only DW's I shy away from are the 22's. I don't really know why the DW concept does not work well in the 22 calibers, but it doesn't. Other than that I would say that the Norwich, NY made guns (current production) and the Monson, MA made guns (first company production) are probably a little, and a stress a little, better in quality control. I have never had a Palmer, MA made gun that gave me any trouble, but the Norwich and the Monson guns are definitely top flight.
1. Check all the usual things like timing, lockup, crane fit and cylinder end shake.
2. Since the cylinder gap is adjustable, if it is adjusted to close, it will make the cylinder hard to turn and sometimes appear out of time when it is not. Just make sure the cylinder face is not dragging against the forcing cone as the cylinder spins. This is a rarely encountered problem and easily fixed in a couple minutes by simply adjusting the gap properly, but I have seen it once or twice in the last 20 years when buying used Dan Wessons.
3. Beware of guns that are assembled with the Power Port barrels. You can tell by looking down the muzzle (with the cylinder open and unlaoded of course). If you see several small holes equally spaced in a ring about 1/2 inch from the muzzle, then it is a Power Port barrel. This was an early attempt at a recoil compensator that did not work very well. Fouling, especially from non-jacketed lead bullets would build up in these ports and make barrel and shroud removal VERY difficult if the Power Port barrel was not removed and cleaned REGULARLY WITHOUT FAIL. If the owner of the gun will quickly remove the shroud to show you it is not stuck, then you are OK on the Power Port issue. Otherwise, beware buying a gun that has a Power Port barrel mounted to the gun. I NEVER use Power Port barrels. They don't shoot any better or worse than the regular barrels, but they require a lot more regular maintenance for no apparent benefit. The newer Compensated barrels, distinguished by 3 slots on top of the shroud just forwad of the front sight, work great and do not have any of the fouling type problems no matter what you shoot in them, with the possible exception of the snake shot loads, which I would not fire in any ported or compensated DW gun.
4. Also beware of the very early and short lived "pork chop" barrel Dan Wessons. This gun is distinguished by the "L" shaped barrel shroud and the protruding barrel nut at the muzzle. I have never heard anything bad about this gun, it is just that none of the modern DW barrels will fit this early model.
5. Also be aware that there was a short period of time when Dan Wesson made fixed barrel guns in addition to the interchangeable barrel design. I believe these were all made in Palmer, MA . So make sure Palmer guns actually have the interchangable barrels if that is important to you.
Every Dan Wesson I ever bought was used. They are so well designed and made that they usually shoot as good as new, unless they have been VERY abused, which is usually obvious at a glance from the appearance of the gun and the simple function tests mentioned above. I hope you find what you are looking for. I hunt deer with a scoped DW 44 mag that is so accurate it is scary.
Roll Tide