I can look at most guns and determine whether they've been fired before or not, unless they have been expertly refinished. When I say "fired" I mean someone putting at least 20 rounds through it. How many people buy a gun and only put 1 or two rounds through it? If we are talking about 1 or two rounds and I can't tell if it's been fired, then I'm not worrying about it.
Anyway, on an exceptionally clean semi-auto that is supposedly NIB, I look at the barrel, particularly where the barrel/slide lock-up. I look at the hammer face, the firing pin. Most new guns will show signs of use in these areas even after a few rounds.
On revolvers, I pay more attention to the hammer face, firing pin, cylinder face, cylinder walls, and forcing cone.
Anyway, on an exceptionally clean semi-auto that is supposedly NIB, I look at the barrel, particularly where the barrel/slide lock-up. I look at the hammer face, the firing pin. Most new guns will show signs of use in these areas even after a few rounds.
On revolvers, I pay more attention to the hammer face, firing pin, cylinder face, cylinder walls, and forcing cone.