I think it's easier to realize one is becoming depressed if they are aware of the basic cognitive distortions such as "I can't do anything right" or "Nothing ever works out for me" type thinking or talking. You can check them out here: http://forums.psychcentral.com/showthread.php?t=108173

I know I'm due for a mind check-up when I begin to think: No good deed goes unpunished. Unfortunately, it's usually due to some small mistake I make here at PC that begins the spiral.

It takes 30 seconds for a thought to become solid. In those 30 seconds it's still changeable ... so when you realize you just thought something negative, change it and the brain will register the positive or the change thought instead. Hang tough.