More specifically, according to a 2014 UK guideline,
A serum folate level of less than 7 nmol/L (3 μg/L) is indicative of folate deficiency;
Red blood cell folate testing is not routinely performed, since serum folate is sufficient in most cases, however, if there is a strong suspicion of folate deficiency despite a normal serum folate level, a red cell folate test may be performed. Plasma total homocysteine is only measured in special circumstances. A level above 15 μmol/L could be indicative of a folate deficiency, but local reference ranges should be taken into account. Prevention and treatment
Diet
Folate is acquired in the diet by the consumption of leafy green vegetables, legumes and organ meats. When cooking, use of steaming, a food steamer, or a microwave oven can help keep more folate content in the cooked foods. Supplementation
Folic acid is a synthetic derivative of folate and is acquired by dietary supplementation. Multi-vitamin dietary supplements contain folic acid as well as other B vitamins. Non-prescription folic acid is available as a dietary supplement in some countries, and some countries require the fortification of wheat flour, corn meal or rice with folic acid with the intention of promoting public health through increasing blood folate levels in the population. Fortification
After the discovery of the link between insufficient folic acid and neural tube defects, governments and health organizations worldwide made recommendations concerning folic acid supplementation for women intending to become pregnant.
5-fluorouracil
hydroxyurea
trimethoprim
sulfasalazine (used to control inflammation associated with Crohn disease, ulcerative colitis and rheumatoid arthritis)
triamterene (a diuretic)
birth control pills (also related to the duration use of birth control pills) may reduce serum folate levels but without  inducing  clinically  significant folate deficiency.When methotrexate is prescribed, folic acid supplements are sometimes given with the methotrexate. The therapeutic effects of methotrexate are due to its inhibition of dihydrofolate reductase and thereby reduce the rate de novo purine and pyrimidine synthesis and cell division. Methotrexate inhibits cell division and is particularly toxic to fast dividing cells, such as rapidly dividing cancer cells and the progenitor cells of the immune system. Folate supplementation is beneficial in patients being treated with long-term, low-dose methotrexate for inflammatory conditions, such as rheumatoid arthritis (RA) or psoriasis, to avoid macrocytic anemia caused by folate deficiency. Folate is often also supplemented before some high dose chemotherapy treatments in an effort to protect healthy tissue. However, it may be counterproductive to take a folic acid supplement with methotrexate in cancer treatment. Cerebral folate deficiency
Cerebral folate deficiency is when levels of 5-methyltetrahydrofolate are low in the brain as measured in the cerebral spinal fluid despite being normal in the blood. Symptoms typically appear at about five months of age. Without treatment there may be poor muscle tone, trouble with coordination, trouble talking, and seizures. The causes of cerebral folate deficiency include mutations of genes responsible for folate metabolism and transport.