  We propose measurements of weighted differences of the angular observables in
the rare decays $B \to K^*\ell^+\ell^-$. The proposed observables are very
sensitive to the difference between the Wilson coefficients
$\mathcal{C}_9^{(e)}$ and $\mathcal{C}_9^{(\mu)}$ for decays into electrons and
muons, respectively. At the same time, the charm-induced hadronic contributions
are kinematically suppressed to $\lesssim 7\% (4\%)$ in the region $1\,$GeV$^2
\leq q^2 \leq 6\,$GeV$^2$, as long as LFU breaking occurs only in
$\mathcal{C}^{(\ell)}_{9}$. This suppression becomes stronger for the region of
low hadronic recoil, $q^2 \geq 15\,$GeV$^2$.
