How do doctors know if i child has a head injury?

We don't...: ...You do, or whoever saw the child get hurt. We do know how serious a head injury may be, by something called the glasgow coma scale: http://www.Unc.Edu/~rowlett/units/scales/glasgow.Htm we use that to decide how to manage the injury. Of course, if there is an obvious wound, that will need to be treated as well.

Vomiting,other signs: If your baby falls and hits his head, and then subsequently has ongoing periods of vomiting, he should be evaluated. Other signs of concern can be: loss of consciousness, lethargy, severe pain/unconsoleable child, loss of movement of an extremity, seizure after fall, loss of developmental milestones, unequal pupils or movements of the eyes. Any of these symptoms require immediate care.

Head injury: Loss of consciousness with a fall or a blow to the head is the most common side of a serious head injury. An associated skull fracture can also be a secondary serious findings. So if a child has a fall with no loss of consciousness or fracture the likely hood of a serious brain injury is low.