You ask how to tell if you have depression. One of the core features of depression is that of a low mood. The other core features that the doctor will look out for is that of an inability to obtain pleasure from activities that would normally give you pleasure (anhedonia).
Other features that would suggest that you are depressed include the following: loss of appetite, loss of weight, poor sleep, decreased energy level, poor concentration, guilt feelings, having negative thoughts like hopelessness, uselessness, thinking that the future is bleak and so on.
For it to be counted as a major depressive episode (MDE), a person will need to have the symptoms for at least two weeks. For a diagnosis of major depression to be made, the individual also has impairment of functioning. That means, school, work or social life is affected by the symptoms.
If you wonder if you have depression, it would be good to seek medical attention. You can arrange to see your family doctor. Alternatively, you can also talk with a mental health professional, like a psychiatrist, a psychologist or a counsellor. It is important to seek treatment early so that you can have a speedy recovery from a bout of depression. Depression can be effectively treated so it is important not to harm yourself or do anything rash. Take good care of your mental health.