  Motivated by the recent hints of lepton flavor universality violation
observed in semileptonic $B$ decays, we analyze how to test flavor and helicity
structures of the corresponding amplitudes in view of future data. We show that
the general assumption that such non-standard effects are controlled by a
$U(2)_q \times U(2)_\ell$ flavor symmetry, minimally broken as in the Standard
Model Yukawa sector, leads to stringent predictions on leptonic and
semileptonic $B$ decays. Future measurements of $R_{D^{(*)}}$, $R_{K^{(*)}}$,
${\mathcal B}(\bar B_{c,u}\to \ell \bar \nu)$, ${\mathcal B}(\bar B \to \pi
\ell \bar\nu)$, ${\mathcal B}(B \to \pi \ell \bar\ell)$, ${\mathcal
B}(B_{s,d}\to\ell\bar\ell^{(\prime)})$, as well as various polarization
asymmetries in $\bar B\to D^{(*)} \tau \bar \nu$ decays, will allow to prove or
falsify this general hypothesis independently of its dynamical origin.
