  We study a weighted asymmetry in the azimuthal distribution of photon-jet
pairs produced in the process p^\uparrow p --> \gamma jet X with a transversely
polarized proton. We focus on the contribution of the Sivers effect only,
considering experimental configurations accessible at RHIC-BNL. We show that
predictions for the asymmetry, obtained in terms of gluonic-pole cross sections
calculable in perturbative QCD, can be tested and clearly discriminated from
those based on a generalized parton model, involving standard partonic cross
sections. Experimental measurements of the asymmetry will therefore test our
present understanding of single-spin asymmetries.
