Make sure you get a gun with removable chokes and that the chokes installed currently are not frozen/rusted into place. Very often people do not clean this part of the gun.
If that area is excessively grimy, or has rust pits etc... the gun was probably NOT well cared-for.
It's pretty hard to kill a double, no matter what you throw at it. Make sure the ejectors / extractors (probably ejectors unless someone converted it or you buy a 'cheap' double - or a very expensive one...) work. Take the forearm off and look there for dirt/grit. Take off the barrel ***'y and look for scoring on the mating surfaces. Look at the hinge pin for the same thing. Any problems here and the gun's a 'don't buy'. Reassemble the gun.
Try dry-firing it. The dealer should offer you caps for this purpose. Fire one barrel and lightly rap the buttstock on the floor. This should set up the second barrel (Citori, other recoil doubles, a mechanical-converted gun will set right away). Fire again. Both firing pins should drop and they should sound the SAME. Feel the trigger creep and release as you do this. It should definately be the same for both bbls. If it's different it's probably because there's a ton of junk in the action and it needs cleaning. This means removing the buttstock.