  The ratio of the Higgs couplings to $WW$ and $ZZ$ pairs, $\lambda_{WZ}$, is a
fundamental parameter in electroweak symmetry breaking as well as a measure of
the (approximate) custodial symmetry possessed by the gauge boson mass matrix.
We show that Higgs decays to four leptons are sensitive, via tree level/1-loop
interference effects, to both the magnitude and, in particular, overall sign of
$\lambda_{WZ}$. Determining this sign requires interference effects, as it is
nearly impossible to measure with rate information. Furthermore, simply
determining the sign effectively establishes the custodial representation of
the Higgs boson. We find that $h\to4\ell$ ($4\ell \equiv 2e2\mu, 4e, 4\mu$)
decays have excellent prospects of directly establishing the overall sign at a
high luminosity 13 TeV LHC. We also examine the ultimate LHC sensitivity in
$h\to4\ell$ to the magnitude of $\lambda_{WZ}$. Our results are independent of
other measurements of the Higgs boson couplings and, in particular, largely
free of assumptions about the top quark Yukawa couplings which also enter at
1-loop. This makes $h\to4\ell$ a unique and independent probe of the
electroweak symmetry breaking mechanism and custodial symmetry.
