It is difficult to tell the difference as both may cause nasal congestion, runny nose and sneezing.
Itching - especially of the eyes, nose, and sometimes the ears and throat is a common feature of allergies, but is usually not present with a flu.
Other symptoms, such as body aches, fever and sore throat, commonly occur with a flu but these symptoms are absent in allergies.
Flu symptoms usually last for a few days and usually resolve within a week while the allergy symptoms may last for as long as the trigger is present.
Like pollen allergy which may last for an entire season or pet allergy which last for the entire time the person is exposed to the particular animal.