  A non-minimal coupling between the dark matter and dark energy components may
offer a way of solving the so-called coincidence problem. In this paper we
propose a low-$z$ test for such hypothesis using measurements of the gas mass
fraction $f_{\rm{gas}}$ in relaxed and massive galaxy clusters. The test
applies to any model whose dilution of dark matter is modified with respect to
the standard $a^{-3}$ scaling, as usual in interacting models, where $a$ is the
cosmological scale factor. We apply the test to current $f_{\rm{gas}}$ data and
perform Monte Carlo simulations to forecast the necessary improvements in
number and accuracy of upcoming observations to detect a possible interaction
in the cosmological dark sector. Our results show that improvements in the
present relative error $\sigma_{\rm{gas}}/f_{\rm{gas}}$ are more effective to
achieve this goal than an increase in the size of the $f_{\rm{gas}}$ sample.
