With any type of head injury, if there has been a loss of consciousness or there is evidence that your athlete has confusion, nausea/ vomiting, visual disturbances or has a headache that will not resolve with a simple dose of Tylenol or Advil, they should be seen by a physician experienced with concussion evaluations. Depending upon the outcome of that evaluation, advanced imaging like a CT scan of the brain may be required.
With extremity injuries, a good role of thumb is if they cannot reliably bear weight on a lower extremity or use their upper extremity for simple tasks like opening a door or carrying a book, it is best to have a physician evaluate that injury and take diagnostic x-rays to rule out the possibility of a fracture. Keep in mind that even though somebody may be able to move a joint, there may still be an underlying fracture and the only reliable method to determine this is with an x-ray and a physician evaluation.