  In the SU(3) symmetry limit, semileptonic $D^+\to Sl^+\nu$ and $B^-\to
Sl^-\bar\nu$ decays, with $S=a_0(980)$, $f_0(980)$ and $f_0(600)$, are found to
obey different sum rules in the $\bar qq$ and the tetra-quark descriptions for
scalar mesons. Thus these sum rules can distinguish the two scenarios for light
scalar mesons model-independently. This method also applies to the $\bar B^0\to
J/\psi(\eta_c) S$ decays. Two kinds of SU(3) symmetry breaking effects are
found to be under control, which will not spoil our method. The branching
fractions of the $D^+\to Sl^+\nu$, $B^-\to Sl^-\bar\nu$ and $\bar B^0\to
J/\psi(\eta_c) S$ decays roughly have the order $10^{-4}$, $10^{-5}$ and
$10^{-6}$, respectively. The ongoing BES-III and the forthcoming Super B
experiments are able to measure these channels and accordingly to provide
detailed information of the scalar meson inner structure.
