How do people know if they are depressed?

Mesonfielde
You know, I was always somewhat wondering how people end up realizing that they might suffer from depression and actually do need medical attention.

Considering depression alters the way you experience the world, if it has been chronic enough for long enough, then how would someone even notice there is something wrong with them?

I would think there is a large chance that after a while, they would just brush it off and say it is just how they are. So how can they tell that they actually have a *problem*?
Updates
+1 y
... anyone who is depressed and figured out they have a medical problem and need medical attention are free to comment...
How do people know if they are depressed?
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