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.

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;
}