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 λ_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

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... at future e^+e^- colliders, such as a 500-GeV ILC with the beam polarization P(e^-,e^+)=(-0.8,+0.3) in the jjℓ^±bb and ℓ^±ℓ^±ℓ^∓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^-→ W^+ W^- H with the measurements of HZZ coupling at the HL-LHC, one can further improve the sensitivity of λ_WZ.