For me... I knew I was sad. I knew I felt crappy, even if I didn't know the reason.
When I was depressed.. I didn't know it until I look back on how I was then. I wasn't particularly noticable.. but *I* Definitely notice it now. It felt like a string of sad days.. I kept blaming this, or that, or saying I'm just moody, etc. I'd get irritated easily.. nothing worked.. it was an attitude I had adopted somehow. I felt like I was walking in the sand, while cussing at people walking on the sidewalk next to me.. I made excuses all the time--well there's already people on the sidewalk, I'd be in the way.. or the sidewalk is too hot and my feet are bare. I'm sick, or my back hurts, so they say I should walk on sand.. .. It wasn't until I noticed the only thing stopping me from walking on the side walk was myself and my lame excuses that I realized where I truly was.