A sure sign that I am depressed is that I am messy. I don't feel like cleaning up after myself.
Another: *dramatic* weight loss. Like 18 pounds in 3 weeks dramatic. (full crash.)
Another sure sign is that I get the 1000-mile stare and become very, very quiet. Depression forces me into more catatonia than anything. When doing something, it is not watching me work, but more of robotics and instinct taking over for me. Very mechanical in functionality.
As for snapping out of it; time. Forcing the issue with me makes me irritable, and forces me even deeper. I have only been seduced out of a funk once, and it took my lover at the time to do it. But that was years ago.