The BABAR, Belle, and LHCb experiments have found that the rates of the radioactive decays B —> D tau nu and B —> D* tau nu are higher than the values predicted by the standard model by 4.1 sigma. This is the largest discrepancy between theory and experiment in particle physics, and therefore must be studied with improved precision. Starting in 2018, precise measurements will be performed at the Belle-II experiment. Operating at the world’s highest-luminosity particle collider, Belle-II’s final data sample will be 35 times larger than those of Belle and BABAR combined. At that precision, thorough understanding of the background will be necessary. In particular, background from B —> D** l nu is not well understood. We present a new, model-independent method for cross-checking the B —> D** l nu background rate. The method utilizes the small beamspot of the collider and the high spatial resolution of the Belle-II detector to detect the impact of the small flight distance of the tau lepton in signal decays, thus making no assumptions about the kinematic characteristics of the B —> D** l nu background.