How do I know my vitamin d level is low?

Testing. There isn't a way to know for sure outside of a blood test. Most individuals at the end of the winter, farther from the equator, little outside activity, or darker skin are likely to have lower vitamin d levels. There are lots of theories on vitamin d levels, and we know that supplementing raises the number, what we don't know is if in the long term supplementation helps or hurts. Good luck.