#1.In pediatricians office- childhood allergy panel (blood test) can be ordered, not 100% accurate, it tests 14-18 allergens.
#2. Dermatologists do skin allergy testing.
#3. Parents can find out by the process of elimination. If 4 or 5 food are suspected, stop all these for 2 weeks, then re-introduce one in each week ...And find out which one is responsible.