Like Beemer-mark says, none of them are literally "unfired". And a gun that's been shot a lot, but meticulously maintained, can be good as new.
One telltale to look for, though, is the condition of the recoil shield. A revolver will fairly quickly start showing the shadow of an image of the cartridge base around the firing pin. The more rounds fired, the more clear it will be. Eventually, there may be an actual, very shallow impression there, and even a bit of erosion around the firing pin hole from minute, repeated primer leaks.
This is no big deal at all, but it can sometimes reveal more shooting than claimed on an otherwise pristine gun.