Suspect a nasal cause of headache if there’s prominent nasal discharge, congestion, or sneezing. Is the headache primarily frontal or facial? Do we see any clues on exam, such as pale nasal mucosa, allergic “shiners” (dark lower eyelids, from venous congestion), or horizontal crease across tip of nose (“allergic salute,” from constant wiping)?
Beware — “sinusitis” is a terribly overused diagnosis which lacks decent clinical gold standards, & generates prescriptions for future antibiotic resistance. When a patient complains of “headache,” if you feel obligated to prescribe something for the nose, call it “allergic rhinitis” and treat accordingly.