I can understand your concern.
yes vitamin D3 can be measured but the only problem is that it is expensive.
if price is not an issue for you then you an get it done by a good reputed lab near you and if you have any deficiency then you should spend more time in the sun and a tablet. D3 must 60k once a week for 2 months. this tablet needs to be chewed rather than swallowed with water.
if the deficiency is more, then you can even take a injection. uprise d3 6L once a week for 3 weeks .
Dr. Anil kumar jain