I am on the same page as Matt, reproduction of period commodities is an unfortunate and unwelcome trend, that probably helps few in the long run. Air gun artefacts are far from alone in being copied however.
I have noticed an ever growing tendency for attractive enamel signs to be reproduced. An example is the black on white Purdey trade sign, of which there were three examples on the last time I looked, all at different prices, all with the 'rust' and 'corrosion' reproduced as well!
All very clever until you find out you have paid ten times the going rate, and find out eventually you will!

Perhaps one saving grace with pellet card boxes is that a pre war box, even if kept untouched in a drawer, will still exhibit signs of aging inevitably after all those years. There will be discolouration of the paper; the corners of the end flap for example will have started to curl, and the box itself will probably be slightly misshapen, with evidence of scuffing at the edges?
You would have to be clever to fake all that, and given the relatively modest value, the work involved wouldn't be worth the time and effort?

Another obvious area to examine, but less easy to do at an arms fair or in someone's front room of course, is to analyse the nature of the ink. Modern inks use different chemicals to those of yesteryear, so with the relevant analysis, immediate confirmation of age, or not, could be realised.

Also of course there is the issue of the pellets inside. They would in all likelihood have started to oxidise, and will have stained the inside of the card packaging accordingly. And if the box is 'mint' but empty, you have to ask the question where did all the pellets go?
In a fake box they either won't be there, or there'll be replaced with newer looking and probably non original slugs. And the inside of the box will therefore be as clean as a whistle.

So whilst I generally deplore the reproduction of anything - I mean people who wear fake Rolex watches to impress, what's that all about - I think if you look hard and ask the right questions you should escape being 'taken in'?