How is folic acid deficiency anemia diagnosed?
Your doctor will examine you and ask questions about your past health and how you are feeling now. You will also have blood tests to check the number of red blood cells and to see if your body has enough folic acid.
The level of vitamin B12 will be checked too. Some people whose folic acid levels are too low also have low levels of vitamin B12. The two problems can cause similar symptoms.
Source: http://www.webmd.com