If you do go down the testing route, you need to be aware that the serum B12 test is hopelessly inaccurate – I honestly don't know why anyone wastes time and money on it. MMA and homocysteine tests will give you a better indication.
If there's any reason you might have low intake (a vegan diet, for instance, or vegetarian without dairy), or become unable to absorb B12 from food (e.g. a history of h. pylori infection or gastritis, or relatives with pernicious anaemia), then I'd definitely take it seriously. If you do have absorption problems, oral supplements won't necessarily work well, though still worth trying in the first instance.