Excessive sadness or feeling down. This can be a sign of depression or depressive stage of bipolar disorder. It also can be caused by prolong drug abuse such as marijuana.
The inability to focus or concentrate at home, work or school can be signs of several different illnesses.
Problem sleeping. This can be a problem with depression or a sleep disorder like insomnia or narcolepsy.
Eating Disorders where the person bings on eating or has a persistent lose of appetite. This is usually accompanied with drastic weight loss and gain.
A sudden and drastic change in personality. This is common with dissociative identity disorder as well as drug abuse and some other mental illnesses.
Withdrawal from daily activities in general especially those that once enjoyed and from family, friends and society in general.
Thoughts of death and of suicide is a good sign of depression but the depression can be a symptom of something else going on.
Extreme or even mild mood swings. Especially if the swing from manic-despressive is in a matter minutes or even moments.
Sudden emotional outbursts such as crying fits or fits of angry for no apparent reason. Also emotional responses that do not match the situation at hand.