My son has a stopped up/runny nose, headache, & upper throat pain with no fever? How can I tell if he is sick or his allergies?

Duration of symptoms. Most upper respiratory infections will last less than 10 days, allergies tend to persist and are frequently associated with sneezing, itching, runny nose and eye redness/tearing. If an antihistamine significantly improves the symptoms, that is suggestive of an allergy. If symptoms persist, an allergist can test to determine if an allergy exists.
Upper resp infection. Most common cause of sore throat, runny nose and headache is upper respiratory infection. Color of mucus not diagnostic but nasal cytology and history of seasonal or exposure triggered events might suggest allergy predisposition.
URI vs allergies. How long has he had symptoms? If only a couple of days, then likely an upper respiratory infection. If this is a recurring or chronic problem then it is likely allergies. Seeing an allergist could help to better define what he might be allergic to.

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