The factory test fires each gun so no gun is actually "unfired" but can still be considered as 100%. After the test firing the gun was cleaned as new (unfired).
Look for power soot around the cylinder front and rear, rear of barrel and around the firing pin. If the gun was originally purchased for defense around the home the gun had bullets loaded and will show bluing wear where the cartridge will rub. If the gun has been carried, it will show signs of holster wear. Look at the front of the barrel, front sight and all sharp edges that would contact the holster.
Look for a cylinder turn line. Be cautious of the re-blue. This would be more concerning than not being 100%. The edges will be sharp, the stamping will be sharp. Take a 10x magnifying glass and look at it outdoors in good light. Indoor lightning can hide a lot of problems. If the seller won't allow the gun to go outside for viewing then he is hiding something. Just walk away.
In my 4 years of collecting, I probably looked at 1000 guns before I found one in MINT condition. That was a early Trooper .22LR with 4" barrel.
Gent's, Keep in mind that I only look at guns that are older than 1965 so mint is harder to come by. When looking for 30, 20 and 10 year old guns, they become much more common.
Let us know how the deal goes for you.