  Understanding the nature of the exotic XYZ resonances is one of the open
problems in hadronic spectroscopy. Despite the experimental efforts, the
structure of these particles still lacks of an accepted theoretical framework.
We propose to use the $Z_c^{(\prime)} \to \eta_c \rho$ decays as a possible
discriminant between two of the most popular models: the compact tetraquark and
the loosely bound meson molecule. We show that the predictions obtained within
the two pictures are significantly different and therefore the proposed decay
channel might shed some light on the nature of these states.
