Given a blood test that contains the levels of parathyroid hormone, vitamin B6 and calcium, how to infer the level of vitamin D?
Exposure to adequate sunlight is a better predictor of vitamin D levels than PTH, B6, and calcium. It's unlikely that a solid enough correlation exists between those and D. If you're rarely exposed to sunlight wearing bathing suit attire during peak UVB hours (11am - 1pm in most of North America), and you also don't take a supplemental form, you're very likely to be on the low side, if not outright deficient.
The only way to know for sure is to have your 25-hydroxy vitamin D levels checked via bloodworm. If you like, you can go here to participate in a study that will let you measure your levels a couple times a year for 60 dollars.
Vitamin D is arguably the most important vitamin to supplement with for most people in developed countries. Now being called a 'master hormone' by some researchers, it controls over 10% of the human genome through epigenetic influences. Correction of vitamin D levels has been associated with improvements in a wide variety of illnesses, from depression to autoimmune disorders to hormonal imbalances.
Optimal dosage is anywhere from 5,000 ius per day to as high as 20,000 ius for some people, though this is excessive for most.