That's not as easy to find out as it seems... There is python-apt, but that is not ported to Maemo as of yet. The nasty part is that you have no info which repo has an app been actually installed from, only which repos contain it NOW, which is not necessarily what you want. Plus, just to add a spin to it, there is good chance repositories will start to get cleaned up, so you might have an app installed whose (installed) version is currently not present in any of the repositories.