Depression is a very complex illness, and there are lots of signs and symptoms. However, in saying that, there are a number of warning signs that you can watch out for, if they persist over a few weeks, and are starting to affect your social, work, and family life, then you are probably depressed.
These signs can be emotional: feeling tearful, not enjoying things that you used to, continuously feeling sad, feeling anxious, thoughts of self-harm or suicide, feeling hopeless, feeling helpless, feeling guilty.
There can also by physical symptoms: lack of energy, loss of interest in sex, unexpected weight gain or loss, difficulty sleeping or waking in the morning, inexplicable aches and pains.
If you are depressed you will find that these symptoms are having a noticeable effect on your life. You may be cutting yourself off from other people, or not doing well at work or school. There are different kinds of depression, and they affect people in different ways. For example, if the symptoms are worse on winter, then you may have seasonal affective disorder.
The most important thing to know is that you can get help. If you are finding depression lasts for more than a few weeks, or a shorter time but you have thoughts of self-harm, then seek medical help. They are lots of support strategies that do not involve taking anti-depressants. I would encourage anyone who feels depressed to speak with their doctor. The sooner you get medical help, then the sooner you can start learning to live life to the full again.
Normally if you feel low for about a month straight and it is interfering in your day to day life you should get some help. There is heaps of cheap or free help available out there. Don't be scared to say you need help, we all need help sometimes.