  We demonstrate that polarization measurements in exclusive semileptonic B
decays are powerful tools for unraveling non-standard model sources of
T-violation. Measurements of the transverse polarization of the tau lepton in
the B to D tau nu and B to D* tau nu decays probe separately effective scalar
and pseudoscalar CP-violating four-Fermi interactions, whereas the D*
polarization in the B to D* l nu (l=e,mu) decay is sensitive to an effective
right-handed quark current interaction. Two T-odd polarization structures exist
involving the D* polarization and they can be isolated and studied separately.
An estimate of these T-odd effects is also given in the context of
supersymmetric theories.
