Here is one of my favorite online resources (Mental Health America) for different screeners for all sorts of mental health conditions (depression, anxiety, PTSD, bipolar, etc.). They are quick, easy and anonymous. You do NOT have to put any identifying information in order to get the results.
The website also has a lot of different resources and tools for all sorts of conditions. Screeners are only meant to be the first step to getting help. Follow-up with a primary physician or mental health provider is hugely important.
Depression looks different for everyone so it's really hard to say there is one key sign of depression. With that being said, depression is a mood disorder so a persistent feeling of sadness or loss of interest characterizes major depression and can lead to a variety of other symptoms. Those other symptoms (outlined below) can sometimes be recognized first, depending on the case.
Mood: sadness not triggered by an incident or specific cause, anxiety, general discontentment, mood swings, and loss of interest in things that used to bring joy
Sleep: either sleeping too much or not sleeping enough, any major change in normal sleep patterns, restless sleep
Physical body: excessive hunger or lack of appetite, any major change in normal eating patters, excessive weight gain or loss, fatigue
Behavioral: excessive crying, easily frustrated or agitated, irritability, social isolation
Cognitive: thoughts of suicide, lack of concentration, slow processing
Suicide is PREVENTABLE. 24/7 free CRISIS LINE- text MN to 741741 to get help!