How to detect a flat area in a mesh?

I'm trying to get the flat surface of a mesh. Basically, I will raycast to the mesh and get a particular point. First thing, I need to get the surface area like a square on the given mesh where I am raycasting, then I need to calculate if that area is flat or not.

I got this below code for detecting at least one flat surface in a mesh.

     bool HasFlatshadedSurface(Mesh m)
     {
         Vector3[] normals = m.normals;
         int[] indices = m.triangles;
         int triangleCount = indices.Length / 3;
         for (int i = 0; i < triangleCount; i++)
         {
             Vector3 n1 = normals[indices[i * 3]];
             Vector3 n2 = normals[indices[i * 3 + 1]];
             Vector3 n3 = normals[indices[i * 3 + 2]];
 
             if (n1 == n2 && n1 == n3)
              return true;
     }
         return false;
     }

But, my case is different where as I am trying to get a particular area which I need to scan and get if it is flat or not. How can I do this?