The decay of the narrow resonance ¯B*0s2→B−K+ can be used to determine the B− momentum in partially reconstructed decays without any assumptions on the decay products of the B− meson. This technique is employed for the first time to distinguish contributions from D0, D*0, and higher-mass charmed states (D**0) in semileptonic B− decays by using the missing-mass distribution. The measurement is performed using a data sample corresponding to an integrated luminosity of 3.0  fb−1 collected with the LHCb detector in pp collisions at center-of-mass energies of 7 and 8 TeV. The resulting branching fractions relative to the inclusive B−→D0Xμ−¯νμ are fD0=B(B−→D0μ−¯νμ)/B(B−→D0Xμ−¯νμ)=0.25±0.06,fD**0=B(B−→(D**0→D0X)μ−¯νμ)/B(B−→D0Xμ−¯νμ)=0.21±0.07, with fD*0=1−fD0−fD**0 making up the remainder.