I'm not sure how they know, but the cats are always ready for their boxes. My husband brought the Petbox in from the porch and almost immediately they both came running and then waited by the box. They are spoiled enough to hope every box is for them, but they were insistent that this one definitely was.
You select your own options for Petbox, or you can be surprised. Since KitNipBox is already a surprise, I went with choosing the selections. There was a little toy that is hard to see in this picture and immediately both cats fought over it. I should have picked out two, but we already had one so I figured we wouldn't need two more. Of course, they have since lost the first one and both wanted this one.
We also got a catnip bird, two bags of fresh catnip, and a bag of treats. Finally there was a kit to grow your own cat grass, which we need because we need something for our male cat to eat other than string and paper. He finds things everywhere that he shouldn't eat so we are hoping he will focus on this (probably not, though, because he's allowed to eat it).