You can but...
If you can get a copy of the Hospital Anxiety & Depression (HAD) scale (NHS direct used to use an online verson which told me to seek immediate medical help!) or Becks Depression Inventory they can be 'good indicators' (Both copyrighted though),
However worth talking it through with a 'professional' are there medical or biological reasons for the 'persistent and sustained low mood'?
Are the social reasons eg; relationships, finaces, debt, bereavement etc...
Is is a cause of substance use (eg, alcohol) , diet, lack of exercise... lack of sleep (newborn children) the list could go on, and therefore the routes and support to tackiling or living with do too.