It's diagnosed through a blood test. As far as I know, you can't really diagnose it accurately in any other way. But in developed countries the rule of thumb on

It's diagnosed through a blood test. As far as I know, you can't really diagnose it accurately in any other way. But in developed countries the rule of thumb on that is: Do you work outside in the sun? Do you eat a lot of fish? Do you take a vitamin D supplement? If you answered "no" to all of these questions then you probably have a vitamin D deficiency. Vitamin D deficiency is very common. >The National Health and Nutrition Examination Survey 2005 to 2006 data were analyzed for vitamin D levels in adult participants (N = 4495). Vitamin D deficiency was defined as a serum 25-hydroxyvitamin D concentrations ≤20 ng/mL (50 nmol/L). The overall prevalence rate of vitamin D deficiency was 41.6%, with the highest rate seen in blacks (82.1%), followed by Hispanics (69.2%). https://www.ncbi.nlm.nih.gov/pubmed/21310306 https://www.ncbi.nlm.nih.gov/pubmed/21310306