Find "flat" areas

Hi everyone! Now I am working at the material preset for the project and due to some specific restrictions I cannot use some default functions and have to invent some solutions.
What I need to do for now is to create somehow the generator which will find areas on the object which are "flat" (or almost flat) and exclude them from mask.
I could do thatusing pixel processor and tangent space normals, but there are some areas where the gradient is presented to compensate surface shhading of the mesh. (Imagine the cube where all edges are soft, but it has a baked normals which make it appear flat). So, using only the tangent normals is not a solution and now my main idea is to utilize the object space normals and make the pixel processor count value of each pixel and compare it with its neighbours, but I have no picture of the final math for now and need some suggestions on how could I do that. 
Or if someone could suggest another way to solve that problem, I'll be very glad.
Thanks a lot to you all!