Persistent sad, anxious, numb, or "empty" mood
Feelings of worthlessness, helplessness, guilt
Feelings of hopelessness, pessimism
Loss of interest or pleasure in hobbies and activities that you once enjoyed
Insomnia, early-morning awakening, or oversleeping
Decreased energy, fatigue, being "slowed down" or feeling sluggish
Increased appetite with weight gain, or decreased appetite with weight loss
Thoughts of self-injury, or attempting to injure yourself
Thoughts of death or suicide, suicide attempts
Restlessness, irritability, nervousness
Difficulty concentrating, remembering things, or making decisions
Persistent physical symptoms that do not respond to treatment, such as headaches, backaches, etc.

Keep in mind that these are only possible signs of depression. They don't necessarily mean you are suffering from depression. There are some physical illnesses which can bring on some of these, and there are some drugs which can produce similar side-effects. This is why it is very important to get help. If you do, in fact, have clinical depression, you can get treatment; and if that is not what's wrong with you, then obviously there is something else, which needs attention.