Tessellation calculation

Writing an article at the moment on performance gain/loss on nontesselated highpoly model vs hightessellated lowpoly model. Currently I’m trying to find a way to see how many polygons my lowpoly model gains after being tesselated. Is there a way to see it or can someone tell me how it is being calculated?
Another question that came along is why doesn’t shadows appear on my model when using displacement? Does it have to do with normals from the object and I need to use normal map for it to work?

The highpoly model to the left and the lowpoly to the right with hightesselation and displacement

Answer to calculating triangle count after tessellation can be found here.

Shadows work out of the box with tessellation. Don’t mistake it with shading, which is what you are referring to. For shading you need normal map.