Your doctor will take a detailed history of your symptoms, and examine the inside of your nose. Usually narrowed nasal passages due to the swelling can be seen, together with a post nasal drip. Children with allergic rhinitis often have tired faces with dark circles under their eyes. This is from the congestion in the nose and sinuses.
If allergic rhinitis is suspected, allergy tests to aero-allergens (allergens in the environment) will be done. The cheapest and most accurate test is a skin prick test.