I think the only reliable way is a blood test.
I’ve done a lot of blood testing. Is it normal to test for vitamins or do I have to ask specifically for a vitamin test?
I’m not sure. 
They normally test for certain vitamins and minerals, but you have to get special requests for others. An online test will never be valid.
Thanks @Ninjastar
I asked for a vitamin and mineral test and never got one.
Did you ask your psychiatrist for a vitamin and mineral test? Or primary care doctor?
I asked my primary care doctor. Well actually I asked the front desk when I made my yearly appointment. Then I asked my primary care doctor at the appointment and she just ran away.
My doctor has a micronutrient test. Is that similar?
What did you find out from the micronutrient test?
Never did it. I think I would have to pay out of pocket.
Yeah, you usually have to pay out of pocket for any tests beyond what your doctor recommends.