VERY difficult to say. first, you need protein and fat to build various substances anyway, so if oyu short on that, but have vitamins and mierals, it still wont work.

then you could have all the food you need, but be out of vitamins or minerals, then it wont work either.

Medication interaction can be a huge problem.

many vitamins have to do with keratine forming; keratine is like, nails and hair and hard stuff.

you have your hair follicle, lets say hair wise, everything is nice and dandy, but you have anemia, (way less red blood cells)

so oxygen does not get to your hair cell, see?

the problem is, those vitamins do not only one thing, but lots of things - and those things all interact with each other.

so you could be very well be out of several vitamins, that have nothig to to with hair, but this will still impede hair building.

a sure sign is multiple problems apart from the hair problem. like, broken ragged nails, fatty skin, pimples, oily hairy, swollen face, slow wound healing, fair of brushing teeth, candida on tongue. bad eyesight, and so much more.

I started with copper, than vitamin d, then vitamin c, then calcium, then magenisum. the magnesium did the trick a bit better.

once you have an imbalance, it goes bad from there, because of the interaction. for example, if you take magnesium with out calcium, you will deplete your teeth and bones from calcium. (if i remember correctly)

with out copper, iron cant be absorbed.

and so on.

so I guess one straightforward way would be bloodwork.