Well, if it's an opened deck, then usually the handling is going to be a big giveaway, unless you've somehow managed to pick up a fake Jerry's Nuggets deck with brilliant handling. But usually, I've heard that the fake ones are shocking to handle.
But there are a couple of clues on the outside of the box, too. The seal isn't like a modern USPCC seal; its sides look similar to that of a stamp, with semi-circles cut into it. And the little wrap-around tag that you use to open the seal is red; if it's clear or anything other than just plain red, then it's a fake. Also, the obvious things like the fact that Jerry's were only printed in red and blue.
And then there's something about there being a little code on the inside tab of the box, so you'd have to use a knife to slice it open to see.