If a gun has only been fired 50 or so rounds it can be hard tell if its NIB- because the gun will look new, and won't show any real noticeable brass marks (rings) to the rear of the ejection port (around the firing pin hole)- if you dump enough rounds through the gun, a ring usually starts to form (brass marks, or it may just start taking off some of the finish). But since guns are test fired at the factory (most)- the guns are shipped a little dirty. So with that said, you'll know if someone put 200-300 rounds through the gun.
Also, take the gun apart, and remove the barrel and spring and just look at the underside of the slide- if there is a "descent" amount of finish removed (especially in different places)- the gun was most likely fired additionally to the factory test (some "minor" finish removal is normal for NIB guns). Just look for overall scrapes and rub marks (on the barrel, slide, frame, even on the magazines). hope this helps.
[This message has been edited by JG (edited April 10, 2000).]