Depends. First a history and physical looking for clues as to why this might be happening. Sometimes questionnaires can be helpful. If there is concern that there is a physical cause then appropriate labs will be ordered. Further evaluation by a psychologist or psychiatrist may be needed if there is concern for add/adhd, autism, etc. There are so many causes that it begins with a history and physical.
Aggression is a . symptom.Prenatal, perinatal & post-natal history of health, growth & development + physical, neurodevelopmental & neuropsychological exams are keys to diagnose cause, + a 3-generation medical & psychiatric family history , socio-familial history, infant & parent temperament & parenting styles. Family Therapy is vital, along with specific Behavioral, Educational, Social Skills & Medical Therapies .