Detect a flat surface on a complex mesh?

I am trying to write a script that can detect, wether or not a given object has a flat surface somewhere on it’s mesh or not.

For instance, a sphere would be a negative, where as a cube would be a positive, as this has at least one flat surface.
Furthermore, a capsule would be a negative, where as a cylinder would be a positive, as the two sides of this are flat.

How do I go about doing this?

Thanks in advance.

Actually all surfaces are technically flat as all geometry is made up with triangles. However if you mean at least 1 flat-shaded surface, then you just have to compare the 3 vertex normals of each triangle. If they are equal, it’s flatshaded. Something like this:

// C#
bool HasFlatshadedSurface(Mesh m)
    Vector3[] normals = m.normals;
    int[] indices = m.triangles;
    int triangleCount = indices.Count / 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 and n1 == n3)
            return true;
    return false;

@Vicas I am trying to find the flat surface of a mesh within a given square or rectangle area inside the mesh. Any suggestions?