There are several good articles on this on the web and in books on the topic. There is a common protocol for handling the firearm in the LGS (watch where your muzzle is pointed!), know how to verify the gun is empty (and DO verify that), request permission before you dry fire. Never dry fire at anyone, and only in a safe direction. A flashlight is handy, a special borelight is handier, and the shop owner may have one to lend you. Look for obvious surface corrosion, pitting, etc. Beyond that, there are some very specific techniques posted in articles and in books to explain exactly how to test the lock-up, the action, the timing, the spring strike, the bore condition, the sights, and the like. No point reproducing them here -- just Google something like "test a used revolver." Read these several times (they're long and detailed) and print them out and bring them with you if need be. Don't be shy about taking your time with your examination of the firearm. It will be easier with experience or practice. Find out what the return policy is; some shops do not accept returns on used guns. Some places will let you shoot it on their attached range before buying. Ask around locally if anyone has input on the reputation of the store. If the person at the gun store is impatient with you or dismissive of your questions, move on.
Happy hunting!
