  We discuss various tests of the factorization hypothesis making use of the
close relationship between semi-leptonic and factorized nonleptonic decay
amplitudes. It is pointed out that factorization leads to truely
model-independent predictions for the ratio of nonleptonic to semi-leptonic
decay rates, if in the nonleptonic decay a spin one meson of arbitrary mass or
a pion take the place of the lepton pair. Where the decay constants of those
mesons are known, these predictions represent ideal tests of the factorization
hypothesis. In other cases they may be used to extract the decay constants.
Currently available data on the decays $\bar B^0 \to D^+\pi^-,\, D^{*+}\pi^-,\,
D^+\varrho^-,\, D^{*+}\varrho^-$ are shown to be in excellent agreement with
the factorization results. A weighted average of the four independent values
for the QCD coefficient $a_1$ extracted from the data gives $a_1=1.15\pm 0.06$
suggesting that it may be equal to the Wilson coefficient $c_1(\mu)$ evaluated
at the scale $\mu = m_b$.
