  We present the 3+1 decomposition of the Simon-Mars tensor, which has the
property of being identically zero for a vacuum and asymptotically flat
spacetime if and only if the latter is locally isometric to the Kerr spacetime.
Using this decomposition we form two dimensionless scalar fields. Computing
these scalars provides a simple way of comparing locally a generic (even non
vacuum and non analytic) stationary spacetime to Kerr. As an illustration, we
evaluate the Simon-Mars scalars for numerical solutions of the Einstein
equations generated by boson stars and neutron stars, for analytic solutions of
the Einstein equations such as Curzon-Chazy spacetime and $\delta=2$
Tomimatsu-Sato spacetime, and for an approximate solution of the Einstein
equations : the modified Kerr metric, which is an example of a parametric
deviation from Kerr spacetime.
