  Recently, LHCb observed a double-charmed tetraquark candidate $T_{cc}^+$ and
claimed its isospin to be zero due to the absence of $T_{cc}^{++}$. However,
the absence of the $T_{cc}^{++}$ in LHCb may be due to their low production in
$pp$ collisions, thus the isospin of $T_{cc}^+$ still needs to be clarified. We
propose an efficient way to investigate the isospin property of $T_{cc}^+$ from
the Dalitz plot distribution of $T_{cc}^+\to D^+D^0\pi^0$. This method can also
be applied to other exotic candidates.
