Vitamin D is often low in people who work indoors, cover most of their skin, or have darker skin and limited midday sun. Testing is useful before you start a high-dose supplement, when bones ache, after a fracture, in pregnancy planning, and when a previous result was low and you want to see if the supplement worked.
Testing every few weeks is rarely useful. Levels move slowly. A repeat after 8 to 12 weeks of supplementation is a common interval, if your doctor agrees.
The test is 25-hydroxy vitamin D. It is a blood draw, no fasting required. Very high doses without a number can push calcium up, which is why guessing is a poor plan.
This article explains how tests are used. It is not a personal diagnosis.