There’s no simple test for figuring out what substance(s) a cat is allergic to and then treating from there. Ramón Almela, DVM, says diagnosing an allergy is often an indirect process that requires time and patience.
Blood test or skin prick test? Neither.”
Source: Tuft’s Catnip, March 22, 2023. Link. Almela outlines the three treatment options currently available for cats with allergies including immunotherapy.