  For a generic Higgs boson, measuring the relative sign and magnitude of its
couplings with the $W$ and $Z$ bosons is essential in determining its origin.
Such a test is also indispensable for the 125-GeV Higgs boson. We propose that
the ratio of the $HWW$ and $HZZ$ couplings $\lambda_{WZ}$ can be directly
determined through the $W^+W^-H$ production, where $H$ denotes a generic Higgs
boson, owing to the tree-level interference effect. While this is impractical
at the LHC due to the limited sensitivity, it can be done at future $e^+e^-$
colliders, such as a 500-GeV ILC with the beam polarization
$P(e^-,e^+)=(-0.8,+0.3)$ in the $jj\ell^{\pm}bb$ and
$\ell^{\pm}\ell^{\pm}\ell^{\mp}jj$ channels. The discovery potential of a
general ratio and the power to discriminate it from the SM value are studied in
detail. Combining the cross section of $e^+e^-\to W^+ W^- H$ with the
measurements of $HZZ$ coupling at the HL-LHC, one can further improve the
sensitivity of $\lambda_{WZ}$.
