  Interstellar clouds can act as target material for hadronic cosmic rays;
gamma-rays produced through inelastic proton-proton collisions and spatially
associated with the clouds provide a key indicator of efficient particle
acceleration. However, even for PeVatron sources reaching PeV energies, the
system of cloud and accelerator must fulfil several conditions in order to
produce a detectable gamma-ray flux. In this contribution, we characterise the
necessary properties of both cloud and accelerator. Using available Supernova
Remnant (SNR) and interstellar cloud catalogues, and assuming particle
acceleration to PeV energies in a nearby SNR, we produce a ranked shortlist of
the most promising target systems; those for which a detectable gamma-ray flux
is predicted. We discuss detection prospects for future facilities including
CTA and SWGO; and compare our predictions with known gamma-ray sources,
including the Ultra-High-Energy sources recently detected by LHAASO. A range of
model scenarios are tested, including variation in the diffusion coefficient
and particle spectrum, under which the best candidate clouds in our shortlist
are consistently bright. On average, a detectable gamma-ray flux is more likely
for more massive clouds; for systems with lower separation distance between the
SNR and cloud; and for slightly older SNRs, due to the time required for
particles to traverse the separation distance.
