It's probably best to have your blood drawn twice: once before taking supplements and one after taking them.

The supplements you may then take are B12, methyl-B12 and methylfolate/-THF.

The last two supplements basically bypass two enzymes (which may be less active due to genetic variation), one of which is most likely to be different in some with BP. B12 is used as a substrate, a raw material, used by the enzyme encoded by the gene which is least likely to be different from most.

The second blood test could then be used to see whether one of these two genes may have a different genotype than most people have.

If there's no (significant) difference in your blood plasma homocysteine levels, genetic or enzymatic tests may be necessary.

Apart from brain damage and (hence) psychosis/mania, there's an increased risk of cardiovascular problems, which most/many with BP have.