  For any vacuum initial data set, we define a local, non-negative scalar
quantity which vanishes at every point of the data hypersurface if and only if
the data are {\em Kerr initial} data. Our scalar quantity only depends on the
quantities used to construct the vacuum initial data set which are the
Riemannian metric defined on the initial data hypersurface and a symmetric
tensor which plays the role of the second fundamental form of the embedded
initial data hypersurface. The dependency is {\em algorithmic} in the sense
that given the initial data one can compute the scalar quantity by algebraic
and differential manipulations, being thus suitable for an implementation in a
numerical code. The scalar could also be useful in studies of the non-linear
stability of the Kerr solution because it serves to measure the deviation of a
vacuum initial data set from the Kerr initial data in a local and algorithmic
way.
