According to the Mayo Clinic, the two main signs of depression are:
Loss of interest and pleasure in activities you used to enjoy.
Feelings of sadness and helplessness.
Additionally, most of these symptoms should be present for most of the day, every day, for two weeks or more:
Trouble thinking, concentrating or making decisions.
Significant increase or decrease in appetite.
Irritability or feeling as if you’re in slow motion.
Sleep problems – too much or too little – or fatigue.
Worthless or guilty feelings.
Decreased interest in sex.
A negative view of yourself and your future, perhaps with thoughts of death, dying or suicide.