  We investigate and compare the possibilities of observing decaying dark
matter (DM) in $\gamma$-ray and radio telescopes. The special emphasise of the
study is on a scalar heavy DM particle with mass in the trans-TeV range. DM
decays, consistent with existing limits on the life time, are assumed to be
driven by higher dimensional effective operators. We consider both two-body
decays of a scalar dark particle and a dark sector having three-body decays,
producing two standard model particles. It is found that the Fermi-LAT data on
isotropic $\gamma$-ray background provides the best constraints so far,
although the CTA telescope may be more effective for decays where one or two
photons are directly produced. In all cases, deeper probes of the effective
operators are possible in the upcoming SKA radio telescope with a few hundred
hours of observation, using the radio synchrotron flux coming from energetic
electrons produced in the decay cascades within dwarf spheroidal galaxies.
Finally, we estimate how the SKA can constrain the parameter space spanned by
the galactic magnetic field and the diffusion coefficient, if observations
consistent with $\gamma$-ray data actually take place.
