A vet I used to work for defined how to differentiate between a cold/influenza and allergies, if it interests anyone.

Cold/flu = Temperature, and none of the antihistamine-type 'cold' remedies work. There is no cure for the flu, treatments are only for symptoms.

Allergies = they tend to show up the same times each year, you don't run a temperature, antihistamines work because they are created to counteract histamine reactions (anti + histamine).

Sue