How Do I Know If It's Depression or Just a Bout of the Blues?

If you're moody and blue and there's a good reason for it, and your moodiness is localized or focused only on that one event and you're able to snap out of it and do what you need to do and snap out of it when something fun happens, then that's an appropriate feeling. If you are moody and blue and you don't know why, or if you're moody and blue and you do know why, but you can't enjoy anything else you have difficulty functioning, what you need to do, and that is sustained for at least two weeks, then you got to start thinking depression.