  In the SU(3) symmetry limit, the ratio $R\equiv\frac{{\cal B}(D^+\to
f_0l^+\nu)+ {\cal B}(D^+\to \sigma l^+\nu)}{{\cal B}(D^+\to a_0^0l^+\nu)}$ is
equal to 1 if the scalar mesons are $\bar qq$ states, while it is 3 if these
mesons are tentraquark states. This ratio provides a model-independent way to
distinguish the descriptions for light scalar mesons . It also applies to the
$B^-\to Sl^-\bar\nu$ and $\bar B^0\to J/\psi(\eta_c) S$ decays. The SU(3)
symmetry breaking effect is found to be under control, which will not spoil our
method. The branching fractions of the $D^+\to S l^+\nu$, $B^-\to S l^-\bar\nu$
and $\bar B^0\to J/\psi(\eta_c) S$ decays roughly have the order $10^{-4}$,
$10^{-5}$ and $10^{-6}$, respectively. The B factory experiments and ongoing
BEPC-II experiments are able to measure these channels and accordingly to
provide the detailed information of the scalar meson inner structure.
