I was just talking about how to tell if paper is plastic coated or not, and I think I may have come up with a way to tell, using oil. My theory is that paper that is not coated with plastic will form an oil spot and oil on plastic-coated paper will just sit there and not stain.
I took a piece of plastic coated thin white paper box (I know pretty definitely that this stuff is plastic coated) and a piece of cracker box cardboard, and put a drop of vegetable oil on each. The cracker box cardboard immediately formed a spot, and the other paper did not. Wish I had taken a photo before the Old Man dumped the pieces of cardboard in the garbage by accident. Oh well.
So, anyway, that's one way of testing cardboard to see if it is plastic-coated or not!
Update: Okay, I realize that I'm not being completely scientific about this, mostly because I didn't try other types of known plastic-coated cereal boxes to see if my theory holds true with that type of cardboard. I'll have to keep thinking on this.