Methods are proposed for measuring the ratio $R^{+/0}=\Gamma ({\rm Y}(4S)\to B^{+}{B}^{-})/\Gamma ({\rm Y}(4S)\to B^0{\overline{B}}^0)$ without assuming isospin invariance in exclusive hadronic decays such as $B\to J/\psi K$. The validity of isospin invariance in various $B$ decays is discussed. Isospin violations are expected to be especially small in inclusive semileptonic decays, so that $\Gamma (B^{+}\to X_c^0{\ell }^{+}{\nu }_{\ell })=\Gamma (B^0\to X_c^{-}{\ell }^{+}{\nu }_{\ell })$ is expected to hold at the subpercent level. Suggestions are made for utilizing this relation. The accuracy of a “double-tag” method for obtaining $R^{+/0}$, such as that used for measuring the $D^{+}{D}^{-}/D^0{\overline{D}}^0$ ratio at the ${\psi }^{\prime \prime }\equiv \psi (3770)$, is also estimated.

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