  Galaxy clusters are among the best targets for indirect dark matter detection
in gamma-rays, despite the large astrophysical background expected from these
objects. Detection is now within reach of current observatories (Fermi-LAT or
Cerenkov telescopes); however, assessing the origin of this signal might be
difficult. We investigate whether the behaviour of the number of objects per
`flux' bin (logN-logF) and that of the stacked signal could be used as a
signature of the dominant process at stake.We use the CLUMPY code to integrate
the signal from decaying or annihilating dark matter and cosmic rays along the
line of sight. We assume the standard NFW profile for the dark matter density
and rely on a parametrised emissivity for the cosmic-ray component. In this
context, the consequences of stacking are explored using the MCXC
meta-catalogue of galaxy clusters. We find the value of the slope of the
logN-logF power law (or the increase of the signal with the number of stacked
objects) to be a clear diagnosis to disentangle decaying dark matter from
cosmic-ray induced gamma-rays. For dark matter annihilation, depending on the
signal boost from the substructures, it is either similar to the cosmic-ray
signal (no boost) or similar to the decay case (large boosts). The shift
between the brightest object and its followers also depends on the signal
origin. For annihilation, this shift and the stacked signal are poorly
constrained because of the large uncertainty affecting the boost. We also
underline that the angular dependence of the annihilation signal is not
universal because of the substructure contribution.
