Depression is an insidious thing. It begins subtly, slowly zapping the strength, will-power, desire, and life, until in the end, there is little of anything left of the person who used to be.
There are the usual symptoms you can get from any website that talks about depression: anxiety, apathy, boredom, general discontent, guilt, hopelessness, inability to feel pleasure, loneliness, loss of interest, mood swings, sadness, or emotional distress. None of these in and of itself are a guarantee of depression, but they do signal that depression is a possible cause. Given that depression is so common, even Google has an immediate call out about the symptoms. Impressive.
The trick to surviving depression is to knowing it's coming, to see the signals before the full episode hits you. Given that the signals are different for every person, that trick becomes tricky indeed.