The only reliable way is to get a vitamin D blood level done. If this is below 10 ng/mL, you are vitamin D deficient. If it is above 30 ng/mL, your immune system is functioning OK. If the level is between 50–80 ng/mL (high normal), your immune system is strongest. Vitamin D toxicity occurs only when the vitamin D blood level is higher than 150 ng/mL.
Because vitamin D is the main driver for a healthy immune system, with vitamin D deficiency you get frequent infections (colds and flus) and you are much more vulnerable to getting Covid-19. However, you also get heart attacks easier and even cancer. A person with vitamin D deficiency gets bone and muscle pains, osteoporosis, chronic fatigue and hair loss. Children get rickets: Vitamin D Deficiency: Know the Signs and Symptoms