if someone's a good manager,that would reflect on his ratings.
Some clear objective indicators for me are 1> Manager is always timely with updates to and from project. 2 > Manager ensures high quality of participation, only accepts good posters, for example, and acknowledges all black lists and bans. 3 > Manager communicates well with constructive feedback. Can a poster improve? Can a poster be warned?
I think the above posters Ideas are good guidelines.
I would add reading through the bounty managers recent posts. You can put together a good picture of their general attitude and professionalism or lack thereof.