It goes without saying that head trauma can cause an “Acute Headache,” due not to brain injury, but rather skin swelling. The pain is felt mostly at the point of injury, and is not getting worse or spreading all over. The patient acts normally, and is not confused. Unfortunately, this can be hard to determine if they’re what we call “altered” (drunk or high).
Sometimes there’s an obvious lump that’s tender, but sometimes the swelling is stuck between the skull and a naturally-tight scalp. The latter hurts more, especially when pressed on. This type of injury is never dangerous, as long as the headache doesn’t get continually worse, or spread all over. Even a fractured skull isn’t dangerous unless there’s a deep wound over it, or if it’s so broken that bone presses downward (there’s no treatment for most fractured skulls; the skull doesn’t need a cast, which would be tough to do anyway).
What matters is brain injury, which we determine by how a person is thinking & behaving, not by their headache. This can be hard to tell if the person is drunk or high. See a brief discussion of Concussion and Post-Concussive Syndrome. Also see Epidural Hematoma for the person who has minor head injury, then starts to get worse later (within 24 hours, rarely 48).
Perhaps the most dangerous and overlooked aspect of acute head trauma is a broken neck, which can paralyze. Picture a person hit on the forehead, blood streaming down their face, everybody screaming. Nobody besides a young infant can lose too much blood from the head or face. The most important component of first aid is to immobilize the neck (may require calming the person). If you can push gently, then more forcibly, on each of 7 vertebrae (neck bones), & there isn’t one in particular that hurts a lot more than the others, there’s no broken neck. This only works if the person isn’t confused or drunk / high, isn’t elderly / fragile, and doesn’t have major injury elsewhere (a broken leg can distract from the pain of a fractured vertebra, though the latter is what kills). If in doubt, call an ambulance.