I don't know which is which but you better not go on an airplane with them. Actually what the h--- is the difference? Can some on explain it so it makes sense.
This is a picture of a typical 8-yard knife pleated kilt:


This is a picture of a 4-yard box pleated kilt:


Both are taken from my web site at http://kilts.albanach.org/details.html where you can read more about the traditional box pleated kilt.

Notice the way the pleats are positioned. In the knife pleat, aka side pleat, they all go the same direction. In the box pleat, each pleat is going in alternate directions, so as to form little "boxes."

Aye,
Matt