How to know if my headache is more than just a simple headache/migraine?

Could be anything: Headaches could be due to simple migraines or major brain tumours. It is difficult to say without more information and examination. It could be a tension headache, a viral infection, due to poor eyesight, meningitis or a haemorrhage. In short you need to see a Dr who will take a history and perform an examination, if needed you will be referred for a CT scan as well.

Duration&intensity: Studies show headache disorders can be evaluated by the location, duration, intensity/severity and associated factors such as the aura and nausea that accompany migraines. History of trauma, surgery, infections, medications, pregnancy/mestruation history for you, and tobacco alcohol and alcohol use. Any fever? Does light bother your eyes? If worst headache of your life go to the ER for full exam.