What are the early warning signs of depression?
Not as easy question to answer.
For starters, the early signs of depression are entirely internal. Lack of motivation. Lack of pleasure. Lack of focus. Maybe lack of sleep. So if you don't share your life with an incredibly astute and empathic person, nobody else is going to have a snowball's chance in Hell of noticing.
Secondly, symptoms of depression are incremental, and creeping. They build like the lengthening darkness builds after the summer solstice: gradually, naturally, so that it doesn't seem for quite some time that anything is different. We are creatures of routine and habit, so it's easier to believe that nothing has changed by subconsciously adapting to these new circumstances than it is to identify a problem and change to resolve it. Change causes pain, right now. Depression also causes pain, but not right away, and since you can build up a tolerance, it doesn't seem so bad. It's easier to ignore it, deny it, forget it.
Of course, that's the amazing thing about depression: it works within the psychic immune system to maintain its own invisibility. It uses our own adaptiveness to hide the destruction that it's wreaking for as long as it can. If and when it's noticed, there's very little chance that the victim can do anything on their own to fix the problem.
When I was first diagnosed as having depression, in my early twenties, I was able to trace the symptoms back over ten years. I didn't know what it was and I didn't get concerned, because it happened so organically I just assumed it was life unfolding in the ordinary way.
The next time I succumbed to its darkness I again tricked myself. I should have known what it was and actively worked to abolish it, but it managed to slide into my spirit and insinuate itself into my life, again without my noticing. We are a prideful and stubborn creature, and we don't like to admit weakness. Depression masks itself as a weakness, so we hide it rather than treat it like the disease that it is. We accept the social stigma at face value and cluck over the sadness of it, rather than recognize it as a strategy of a willful, malevolent disease that will do whatever it takes to inculcate itself into our lives.