To my knowing, the most widely used and accurate methods are HPLC (high performance liquid chromatography ), LC-MS (Liquid Chromatography Tandem Mass Spectrometry).
A pretty up to date summary should be in this book:
Eitenmiller, R. R., Landen Jr, W. O., & Ye, L. (2016). Vitamin analysis for the health and food sciences. CRC press.
The world of micronutrients still has many gaps, and the specific case of vitamin b12 is particularly complex given the clinical importance of its levels, which are low, but also high. In this sense, in complement to the debate question, I want to share with you the following manuscript detailing the aspects associated with high levels of vitamin b12.
Article Hipervitaminosis B12 una mirada desde la atención primaria