The clinching data come from looking at how the intensity of the gamma-rays changes as the temperature is altered. Electron-positron annihilation should be more rapid in Ps2 than in lone positronium atoms, because the binding increases the chance of collision. And the positronium mix should have a greater proportion of molecules at lower temperatures, since the cold makes molecules more stable. So the gamma-rays should become more intense when the mixture is cooled. That's exactly what the researchers saw, they report in Nature.
The exact citation for this paper is:
D. B. Cassidy and A. P. Mills, Jr., Nature v.449, p.195 (2007).