With thanks to Les Posen, I can recommend a nice, well devised tool for assessing depression that can be found on
It is The Center for Epidemiologic Studies Depression Scale (CESD), which was created in 1977 by Laurie Radloff, 1 and revised in 2004 by William Eaton et.al.
The site has information about the scale, about the scores and about depression (short, but with links). It also has an option for e-mailing the treating professional one's results. As it asks about symptoms within the last week, it strikes me as a nifty tool for monitoring severity of depression over time.