  Propagating perturbations have been observed in extended coronal loop
structures for a number of years, but the interpretation in terms of slow
(propagating) magneto-acoustic waves and/or as quasi-periodic upflows remains
unresolved. We used forward-modelling to construct observational signatures
associated with a simple slow magneto-acoustic wave or periodic flow model.
Observational signatures were computed for the 171 {\AA} Fe ix and the 193
{\AA} Fe xii spectral lines. Although there are many differences between the
flow and wave models, we did not find any clear, robust observational
characteristics that can be used in isolation ( i.e. that do not rely on a
comparison between the models). For the waves model, a relatively rapid change
of the average line widths as a function of (shallow) line-of-sight angles was
found, whereas the ratio of the line width amplitudes to the Doppler velocity
amplitudes is relatively high for the flow model. The most robust observational
signature found is that the ratio of the mean to the amplitudes of the Doppler
velocity is always higher than one for the flow model. This ratio is
substantially higher for flows than for waves, and for the flows model used in
the study is exactly the same in the 171 {\AA} Fe ix and the 193 {\AA} Fe xii
spectral lines. However, these potential observational signatures need to be
treated cautiously because they are likely to be model-dependent.
