Diagnosis is made by the physical exam, the time of year plus the breed of dog. Breeds such as Pit Bulls, Retrievers, West Highland Terriers, Shar-Pei's, English Bulldogs and others are predisposed to allergies via a genetic path. In many parts of the country it is cold in fall in winter. Most plant allergens and insects are dormant. Noticing an itchy dog in the winter is either dry skin, food allergy or the Cheyletiella mite in puppies. In South Florida, where I live, it never gets cold so insect and tree allergens are a constant year round problem. Surgical skin biopsies, using a biopsy punch, may be taken to rule out other non allergy causes of skin irritation.