  Measurement of $CP$-violating observables in semileptonic decays is a
sensitive null-test of the Standard Model: any $CP$ violation would be an
unambiguous sign of New Physics effects. The model-independent technique to
measure parity and $CP$-odd observables in the $B\to D^{\ast}\mu\nu$ decays is
proposed, which effectively cancels out parity-even terms in the decay density
together with the associated theory uncertainty. The feasibility study is
performed with pseudoexperiments, and the sensitivity at the LHCb experiment is
estimated. Finally, the most significant systematic effects and the data-driven
ways to control them are considered.
