The most important part of diagnosing an allergy is a detailed history: what symptoms you get, how soon after exposure they start, how long they last, and what seems to trigger them. A clear pattern, such as wheezing and sneezing every spring or hives within minutes of eating shellfish, often points to the cause before any testing is done.
To confirm specific allergies, a doctor may arrange a skin prick test, where tiny amounts of allergens are placed on the skin to see if a small welt forms, or a blood test that measures allergy antibodies (specific IgE) to particular triggers. In some cases, usually in specialist settings, a supervised food challenge under medical observation is the most accurate way to confirm or rule out a food allergy. Results are always interpreted alongside your symptoms, because a positive test alone does not always mean a clinically important allergy.
Be cautious of commercial tests sold online, such as IgG food panels or hair analysis, which are not scientifically validated and can lead to unnecessary food restriction. If you think you have an allergy, see a doctor who can take a proper history and arrange reliable testing or refer you to an allergy clinic.