  Using factorization we determine the allowed domains of the ratios of form
factors, $x = A_2(0)/A_1(0)$ and $y = V(0)/A_1(0)$, from the experimentally
measured ratio $R_h \equiv \Gamma(D_s^+ \ra \phi \rho^+)/\Gamma(D_s^+ \ra \phi
\pi^+)$ assuming three different scenarios for the $q^2$-dependence of the form
factors. We find that the allowed domains overlap with those obtained by using
the experimentally measured ratio $R_{s\ell} = \Gamma(D^+_s \ra \phi \ell^+
\nu_{\ell})/\Gamma(D^+_s \ra \phi \pi^+)$ provided that the phenomenological
parameter $a_1$ is $1.23$. Such a comparison presents a genuine test of
factorization. We calculate the longitudinal polarization fraction,
$\Gamma_L/\Gamma \equiv \Gamma(D_s^+ \ra \phi_L \rho^+_L)/\Gamma(D_s^+ \ra \phi
\rho^+)$, in the three scenarios for the $q^2$-dependence of the form factors
and emphasize the importance of measuring $\Gamma_L/\Gamma $. Finally we
discuss the $q^2$-distribution of the semileptonic decay and find that it is
rather insensitive to the scenarios for the $q^2$-dependence of the form
factors, and unless very accurate data can be obtained it is unlikely to
discriminate between the different scenarios. Useful information on the value
of $x$ might be obtained by the magnitude of the $q^2$-distribution near $q^2 =
0$. However the most precise information on $x$ and $y$ would come from the
knowledge of the longitudinal and left-right transverse polarizations of the
final vector mesons in hadronic and/or semileptonic decays.
