The pathology of pernicious anemia is quite different from deficiency as such. It is believed to be due to an autoimmune attack on the gastric mucosa. In addition, as Sayan Paul mentioned, the autoantibodies involved in the pathogenesis are of three types.
The diagnosis of pernicious anemia is based on-
Leukopenia with hypersegmented neutrophils,
Low serum B12
Moderate to severe megaloblastic anemia
Elevated serum levels of MMA and homocysteine.
In Anemia due to folate deficiency, serum methylmalaoate levels are normal and neurologic changes does not occur.
Reference - Robbins and Cotran Pathologic basisi of disease 9th edition - Page 647.