Bronchial asthma can be diagnosed by:

Clinical features: Typical symptoms include recurrent episodes of wheezing, chest tightness, breathlessness and cough. Asthma is commonly mistaken for a cold or a persistent chest infection (e.g. longer than 10 days). Classical precipitants include exercise, particularly in cold weather, exposure to airborne allergens or pollutants, and viral upper respiratory tract infections. Wheeze apart, there is often very little to find on examination. An inspection for nasal polyps and eczema should be performed. Rarely, a vasculitic rash may suggest eosinophilic granulomatosis with polyangiitis (formerly known as Churg–Strauss syndrome). Reference: : Davidson P R I N C I P L E S   and Practice O F M E D I C I N E 23rd edition page no  568,570