One strategy used to measure depression is to look at the physical traits that are around the person.
*How organized is their home, bedroom, or office? Is it typically this way?
Was it this way before they were depressed?
*Are they keeping up on their physical hygiene? Do they remember the last time they showered or brushed their teeth?
*Is the vehicle they drive being regularly maintained?
*How is their yard or balcony? Are indoor plants being cared for?
By looking at the physical surroundings, you can start to get an idea for the internal state of the person. If the person is recently depressed, you can compare recent behavior to past behavior. The cleanliness and completed tasks are a way to measure varying levels of depression; typically the more disorganized and undone things are, the deeper the depression may be.
For me personally, I know I am feeling the best when my yard is cared for, my children are clean, and my dishes are done. Some other things may be falling apart (I can't be perfect!) but these three areas are key peices of insight to my mental health.
I also find that the previous areas (yard, children, and dishes) are coping strategies for me. I enjoyed reading Ginger's blog today on how she used her coping skill of nature (inSERIOUSLYsane: The Healing Power of Nature).