re tests, yes to serum d3, yes to serum b12.
less common to date but definitely yes re serum magnesium (especially if d3 is low and requires supplemental replenishment)
other low hanging fruit you may already have on file and that would be good to know in advance of any supplementation programme:
serum ferritin, hemoglobin.
less common stuff but very relevant and useful info: serum zinc and serum copper (no not urine ceruloplasmin)
nice to know but not need to know: serum uric acid
otherwise, msdx time is definitely learning curve time. think about taking on these simple questions one by one:
early baby steps:have any of your docs ever referred you to a preventive public health professional eg a registered dietitian?
do you have a copy of public health recommendations for diet and essential nutrients?
is there a printout of the harvard healthy eating plate on your fridge?
can you readily access recommended daily intakes of essential nutrients for your age and gender, via a file or link?
do you need to learn what those recommendations are and where you can find them?
do you have a sense of the most common nutrient issues?
those most commonly associated with chronic illness?
with ms in particular?
do you know how closely your day to day adheres or doesn't to existing evidence-based public health recommendations for dietary intake of foods and essential nutrients?
do you need to keep, or have you recently kept, a detailed diet diary of foods fluids supplements and meds? (if not, you can find a selection of diet diary templates online)
once you have thought about all of these questions, the first next steps on a list of action items will likely be obvious
if the answer to 1. is no, ask for the referral.
if the answer to 2. is no, find them and print or bookmark copies (check links at the start of the diet forum)
hope that helps