you can tell on some models, on my KDX it has stamped on the outside of the intake flange on the jug .197 L which is 197cc's. I'm not sure if the KX would have this but it's a place to start.
But isn't it the same exact as the KX 80 Bigwheel? I mean, you can't really tell the difference, right? If you bought a big wheel kit for the KX 80, it would like the same as a KX 100, wouldn't it?