You really don't know. From a collector value stand point, if you can't tell, the the value is high. From a shooter point, the issue is not really unfired, but rather fired very little so that practically the gun has its whole life left.

Note that many guns are function fired, proof fired, and, on top end guns, fired for patterning. Proving a gun hasn't been fired since leaving the factory is most difficult and always open to question.