How to tell of gluten allergy/celiac's disease?

I understand that gluten is hard to digest for a lot of people and most do well on a gluten free diet. My family has been seeing a nutritionist and he has diagnosed my younger sister as having a "gluten allergy" from a spit test, against my better judgment, everyone has gone on board with the gluten free diet and she feels a little better, yet doesn't really feel a difference. I was wondering if the nutritionist can actually diagnose a gluten allergy, or should she see an actual doctor/allergist?