A doctor can simply indicate ‘MTHFR gene’ on the lab test request sheet. Many labs around the country are able to do this test now, and many health insurance plans do cover in this test.
Your doctor may suggest, based on your symptoms, that you simply try the treatment for the MTHFR defect, and decide whether or not it makes a difference. Considering the full set of variants are not visible with the currently available blood tests and that other medical conditions and medications may be restricting methylation (see What is MTHFR? and MTHFR and Methylation), for some individuals a treatment trial is more cost-effective and beneficial. It’s a very benign treatment in terms of side effects because it is just the ‘activated forms’ of essential vitamins, so there is a big upside and not much to lose (see our dosage information if you opt to go this route).
Since you can get both Methylfolate (L-MTHF) and Methylcobalamin (B-12) over the counter, you stand to gain a lot by just trying it to see how you feel and whether it makes a difference for you or not. Because individuals are all different, you could notice a dramatic difference in as few as 3 days or you may notice a more gradual improvement over a couple of month's time.
Here’s one doctor’s opinion on testing: https://mthfr.net/mthfr-screening/2011/09/29/