There are lots of sites on the internet where you can read more about the symptoms of depression. These may give you an idea if this is what you are suffering with. However, a medical professional is the only way to get a definite diagnosis. It's also a great step towards getting the help that you need.
Usually you will know you have depression when you have some of the symptoms such as sleep problems, irritability, loss of interest in activities, etc but the only way to confirm you have depression is to get a medical diagnosis.
Well, they have depression type quizzes you can take on line to let you know if you need to make a appointment to see a therapist. It is very easy to take these quizzes, they usually can be taken within minutes. The questions are not to difficult to answer they just ask you questions like how do you feel at the moment. Do you feel like hurting yourself or do you want to hurt somebody else. If you can answer no to the majority of the question you probably want need to go any farther as in talking to a license therapist. However, you need to be as honest as you can so that you might be able to get the help you need.