to be perfectly honest
if I was not sure it was "new"
field strip it, with the slide removed
look around the groves on the inside of the slide where the firing pin is
its one of the most common places to miss metal shavings when cleaning a pistol. you can tell if its been shot.
now just for what its worth, you must understand that it has been fired even if new, they have to test fire the darn thing before it leaves the factory in New Hampshire. and I have personally purchased a few new from the factory Sig Sauer Pistols, and the barrel always has a wear pattern on the nitron on the outside of it. Dont panic, it is new, its just the way the make the parts. I unwrapped the paper seals on all mine and they all had almost identical wear patterns in the nitron on the upper barrel