However, if you cannot afford to pay for a B12 test or persuade your health professionals to do one, then it's no big deal using a Methylcobalamin B12 spray or sublingual.
If you start using B12 and find you've a lot more energy suddenly then you can be pretty sure your deficient and need to continue.
If you don't notice any difference and are consuming High B12 food sources then you could just take it a few times each week as insurance.
If I was on Metformin though I'd supplement with B12 irrespective of test or other food sources. In the same way I think everyone taking a statin should simultaneously also take CoQ10. I don't understand why this isn't the law. What about the principle "First do no harm"?