I've suffered from depression in the past. Some major signs are:
1) Lack of energy, or being tired all the time.
2) Lack of motivation to do go out and do anything.
3) Inability to enjoy those things that you use to enjoy.
These are not the only symptoms of depression, but are huge warning signs.
If you suffer from depression, all is not hopeless, despite the fact that it feels that way. Attitude really is a state of mind, and if you focus your mind on being happy, you can actually find happiness in your day to day lives. I know it seems somewhat stupid, but the act of wanting to be happy and making an effort to feel happy can actually have a positive influence on your life.