They should look for abnormal behaviour. If a child has fallen, then maybe they're crying irrationally or not eating. There's a change from their normal behaviour.
The problem with concussion and brain injury is that if you've met one person with a brain injury, you've met one person. If you suspect that your child has a brain injury, then visit your physician. However, we need to get that information into people's hands more widely so that they know to look out for this.