For context, I should point out I have dysthymia. So I'm always low grade depressed or at least prone to depressive thoughts and emotions. I have bouts of severe depression that can last days, or weeks, or in a handful of cases a few months.
Bad episodes are either abrupt due to some situational cause or it's an awful slide down that I can see coming and sometimes can't fend off.
In the latter case: frustration, irritability, anhedonia, anxiety, rumination, fading of hope or at least motivation, self esteem gets worse, failing to get things done, increasing tendency to isolate and avoid, increasing emotional volatility. Any or all of these.
A lot of it is the characteristics of the actual depression itself, but resurging and increasing in intensity if I can't reverse it.
So, not hard to spot really, for me.
Oh, and welcome to PsychCentral.