some signs can be
-not wanting to be around people
-feeling tiered or not wanting to do anything
-not finding fun in thing you used to find enjoyable
-extreme sadness all the time (being depressed)
-suicidal thoughts
-cutting oneself
-questioning if life is really worth living.
from someone who experiences depression daily I feel I have covered a few points. there is much more that goes on but here are some of the basics.