As a rough guide, if you honestly feel you can say yes to 5 or more of these in the past fortnight, you may be depressed and should visit your GP.
If you are suffering from just a couple, but badly enough to be affecting your life, I'd still suggest you visit GP.
A persistent sad, anxious or empty mood
Sleeping too little or too much
Reduced appetite and/or weight loss or gain
Loss of interest in activities once enjoyed
Restlessness or irritability
Physical symptoms that don't respond to treatment (such as headaches, stomach upsets)
Difficulty in concentration or with memory or making decisions
fatigue or loss of energy
feeling guilty, worthless or hopeless
Feelings or thoughts of suicide, death or self harm