Most re-sizing dies for bottleneck rifle cases don't actually full length perform a re-size to the same dimensions as the factory ammunition was made at. So you can usually see, on the outside, a slight difference about one eight inch up from base of the case. Where the solid head transitions to the actual walls of the case.
This is because the re-sizing die doesn't actually size down that far and leaves that part ever so slightly a different diameter after the re-size. Often seen as a almost "ring" or shadow on the brass. Once fired brass that hasn't been re-sized won't show that tell-tale in such a sometimes very obvious appearance. It's why, BTW, so named "small base dies" and "extra small base" exist as they do size down that little bit more which is essential for some pump or auto loading sporting rifles.
The other tell tale is that some, not all, makers have a primer that often is sealed with a coloured ink. Or that is, as was Norma, actually impressed with NP on the primer. Or the primer in factory ammunition is usually (Federal, I think, for example) of one colour...yellow metal...whereas the spent primer that is now in that Federal case is white metal.
And, of course, on factory ex-military cases with crimped in primers it'll be obvious that such crimp has lost its original integrity as the original primer has been replaced by (a now spent) primer that has altered the original shape or form of that crimp.
Why is once fired more desirable than many times fired? Because with once fired the buyer knows that the case hasn't been worked and worked and worked again such that it has lost its elasticity (and so liable to splitting soon) and also that the primer pockets will still be nice and tight and not becoming loose.
