  Collisions between giant molecular clouds are a potential mechanism for
triggering the formation of massive stars, or even super star clusters. The
trouble is identifying this process observationally and distinguishing it from
other mechanisms. We produce synthetic position-velocity diagrams from models
of: cloud-cloud collisions, non-interacting clouds along the line of sight,
clouds with internal radiative feedback and a more complex cloud evolving in a
galactic disc, to try and identify unique signatures of collision. We find that
a broad bridge feature connecting two intensity peaks, spatially correlated but
separated in velocity, is a signature of a high velocity cloud-cloud collision.
We show that the broad bridge feature is resilient to the effects of radiative
feedback, at least to around 2.5Myr after the formation of the first massive
(ionising) star. However for a head on 10km/s collision we find that this will
only be observable from 20-30 per cent of viewing angles. Such broad-bridge
features have been identified towards M20, a very young region of massive star
formation that was concluded to be a site of cloud-cloud collision by Torii et
al (2011), and also towards star formation in the outer Milky Way by Izumi et
al (2014).
