Blood work?
Your body accumulates a store of vitamin D in the liver, which it rations out into your bloodstream over time. When you ingest or produce vitamin D, your liver buffers the amount that ends up in your blood, which does two things. 1) saves for a rainy day (literally and figuratively) 2) protects you from vitamin d toxicity over the short term. Which I’m told is quite nasty.
With a doctor’s supervision you can take large doses of vitamin d for a short time and your liver will soak it up. How fast your blood levels rise tells the doctor about how much more you need in the tank and/or if you are having absorption problems. They taper you off quickly as you approach a healthy blood level.