  We perform 3+1D viscous hydrodynamic calculations of proton-lead and
lead-lead collisions at top LHC energy. We show that existing data from
high-multiplicity p-Pb events can be well described in hydrodynamics,
suggesting that collective flow is plausible as a correct description of these
collisions. However, a more stringent test of the presence of hydrodynamic
behavior can be made by studying the detailed momentum dependence of
two-particle correlations. We define a relevant observable, $r_n$, and make
predictions for its value and centrality dependence if hydrodynamics is a valid
description. This will provide a non-trivial confirmation of the nature of the
correlations seen in small collision systems, and potentially to determine
where the hydrodynamic description, if valid anywhere, stops being valid.
Lastly, we probe what can be learned from this observable, finding that it is
insensitive to viscosity, but sensitive to aspects of the initial state of the
system that other observables are insensitive to, such as the transverse length
scale of the fluctuations in the initial stages of the collision.
