This is one of the most common email queries.
Please do have a look around the site and in particular, use the symptoms calculator to check out your symptoms against known B12 deficiency symptoms - that will tell you if you have B12 deficiency or probably have it.
For example, there are certain common conditions which are always or nearly always associated with B12 deficiency, and in some cases, simply taking B12 (or having B12 from injections) is sufficient. Examples include:
B12 is a very fundamental vitamin. Usually you will have more than one of these in order to diagnose B12 deficiency