Usually extramural testing (by "experts" or "specialised expert institutions" is recommended:
cf: Intradermal skin tests in equine dermatology: a study of 83 horses.
Lebis C1, Bourdeau P, Marzin-Keller F. in: Equine Vet J. 2002 Nov;34(7):666-71., @ https://www.ncbi.nlm.nih.gov/pubmed/12455836
cf: A comparison of intradermal testing and detection of allergen-specific immunoglobulin E in serum by enzyme-linked immunosorbent assay in horses affected with skin hypersensitivity. by Morgan EE1, Miller WH Jr, Wagner B. in: Vet Immunol Immunopathol. 2007 Dec 15;120(3-4):160-7. Epub 2007 Aug 19. @ https://www.ncbi.nlm.nih.gov/pubmed/17888519
Blood=Serum for allergy testing (also known as “serum IgE” testing.) @ https://www.acttallergy.com/allergy-facts/equine-allergies/testing-treatment/
"There are two tests currently available for food allergy testing in horses: intradermal testing (known as a skin-prick test) and a blood test...": cf @ www.equinews.com/article/food-allergy-testing-horses