Checking if a Transform/Matrix is Mirrored

I was wondering how to determine if a Matrix is mirrored along some axis.

I have a way in mind but I was wondering if there is a better way to do it:

 bool mirrored = Vector3.Dot( transform.up, 
                              Vector3.Cross( transform.right, transform.forward )) 
                 > 0 ? false : true;

Thanks, in advance.