  In semi-leptonic $B$ decays with a tau lepton, features of the production
process are imprinted on the tau helicity states. Since the tau momentum cannot
be fully reconstructed experimentally, the available information on the tau
properties is encoded in its visible decay products. Focusing on the process
$B\to D\tau\nu$, we find explicit relations between the tau properties and the
kinematics of the charged particles in the decays $\tau\to\pi\nu$,
$\tau\to\rho\nu$, and $\tau\to\ell\nu\bar{\nu}$. In particular, we show that
the perpendicular polarization, $P_\perp$, and the forward-backward asymmetry,
$A_\tau$, of the tau lepton can simultaneously be extracted from an angular
asymmetry of the charged particle against the $D$ meson. For the most sensitive
decay channel, $\tau\to\pi\nu$, we expect a relative statistical precision of
about $10\%$ for $P_\perp$ and $A_\tau$ in a measurement based on
$50\,$ab$^{-1}$ of data at BELLE II.
