  We develop a phenomenological theory to predict the characteristic features
of the momentum-dependent scattering amplitude in resonant inelastic x-ray
scattering (RIXS) at the energy scale of the superconducting gap in iron-based
superconductors. Taking into account all relevant orbital states as well as
their specific content along the Fermi surface we evaluate the charge and spin
dynamical structure factors for the compounds LaOFeAs and LiFeAs, based on
tight-binding models which are fully consistent with recent angle-resolved
photoemission spectroscopy (ARPES) data. We find a characteristic intensity
redistribution between charge and spin dynamical structure factors which
discriminates between sign-reversing and sign-preserving quasiparticle
excitations. Consequently, our results show that RIXS spectra can distinguish
between $s_\pm$ and $s_{++}$ wave gap functions in the singlet pairing case. In
addition, we find that an analogous intensity redistribution at small momenta
can reveal the presence of a chiral $p$-wave triplet pairing.
