Generally speaking, close inspection of high-wear surfaces will tell the story here, unless the gun is used, but essentially unfired. While this seems like common sense, there are a few variables to consider: for example, different guns show early wear in different places, and some manufacturers seem to do a lot more proof testing at the factory than others.
In autos, look at the breechface, hammer, ejection port and magazine well for starters. (If there is more than one example of a particular model in the display case, you may ask to see a few of the others for comparison.) If the gun has fired more than 100-200 rounds, you should definitely be able to seel evidence of it in these areas; if not, then the gun is probably worth the money no matter how it's being advertised. Since condition is everything, a pristine gun that may have been previously "titled" will still retain its original value.
The difficulty in making this call helps to explain (at least in part) why most gun shops will ask you to refrain some cycling the action or dryfiring unless you appear to be on the verge of a purchase.
Chuck