How we will come to know that person has allergy ?

Symptoms: If you develop certain symptoms when you are in contact with something (cat,dust , drug etc) then you likely have an allergy to it. The symptoms depends on which parts of your body have been affected. Nose-sneezing, runny nose, stuffy nose; lungs-wheezing, breathing problem, coughing; eyes-watery and red. Distinct symptom is itching (not in lungs). Skin or blood test can confirm the presence.