there are lots of signs and symptoms or major depression which should be present for at least 2 weeks
1- feeling low mood most of the day for almost every day
2-feeling of worthlessness
3-loss of interest in daily activities that used to interest you.
4- not eating or over eating
5-not sleeping or over sleeping
6-agitation or restlessness or crying spells
7-inabitlity to concentrate
8-fatigue or loss of energy
if you or any close friends notice at least 5 of the above criteria (with at least the 1st or 2nd)with no possible cause (like grieving...) you need to see a psychiatrist as soon as possible.