Get tested.
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Allergies have very specific symptoms. "People assume that if they have a runny nose or some other symptom, like a rash, that's an allergy," says Riedl. "But there are lots of reasons to have runny noses and lots of reasons to have rashes." If you think you may have allergies, look at all of your symptoms and take note of when they arise. For example, runny nose and sneezing aren't the only symptoms of outdoor allergies caused by pollen and mold spores – the allergies are also marked by itchy, watery eyes. So if you live in the Midwest and experience these symptoms a lot during ragweed season (August through October), you may be allergic to ragweed pollen

Your best bet is to get tested by a doctor. Two types of tests are available: blood tests that detect different IgE antibodies, and skin tests that look for reactions after exposing the skin to various allergens. Don't depend on at-home blood tests, which may produce false positives – people do develop IgE antibodies without having allergic reactions, so it's difficult to get an accurate diagnosis without discussing your actual symptoms and family history with a doctor.