How does a person know surely that they are...

Depression is a psychological disease that appears as a control of negative thoughts on the patient, feelings of sadness, frustration, and lack of ability to enjoy life
Symptom of this disease
Isolation and avoiding social contact with people
Sleep disorder (restlessness or too much sleep)
Loss of Apetite
Feeling sad and constant frustration even in attempts by those around the patient to alleviate the status
Feeling of insignificance in life and low self-confidence
Anxiety and anger are quicker than the trivial things, as well as feeling anxious of usual things
Feeling of hopelessness, helplessness, depression and constant crying
Losing interest in things you used to love and enjoy
Complaining about pain in the body without an organic cause
Feeling tired from the least effort
Mood swings and anger
Finally, the symptoms worsen, leading to suicidal thoughts.
Therefore, if you notice the symptoms you have for a period of not less than two weeks, it will be an indication of depression, and hereby, you are recommended to follow up with the psychiatrist to prescribe drug therapy in addition to cognitive behavioral therapy, which aims to change the negative lifestyle and help you get rid of defeatism and learn methods to improve social skills and ways to confront pressure
Psychological therapy helps you to understand emotional conflicts, especially conflicts with painful experiences in childhood, and it also helps to solve with conflicts and deal with loss, separation or acclimatization to life.
Find below, some tips to alleviate grief
Facing your problems and looking at them as they are not constant and will end soon, however long or short
Reject your problems and concerns with a close person or by writing on paper
Exercise and do relaxation exercises, which will help you eliminate negative energy and reduce anxiety
Go out for a walk and get in touch with friends
Practice your hobbies and do all the things you like 

You have ultimately to organize your life and make a daily plan tips for yourself for all the tasks that you wanna do, as this will greatly reduce your stress