Red flags: age > 50, worse headache of life, acute/thunderclap onset, fever or systemic sx, immunocompromised, known cancer/cancer w/brain met potential, altered mental status, worse w/cough or activity, worse w/valsalva, post head trauma; new neurological deficits (by hx or exam). Note that even chronic headaches (tension, cluster, migraine) will have a first time/acute presentation. Also note that acute headache sx that are different than typical HA (for those w/chronic HA) is a red flag.