Children with autism have trouble with basic communication, prefer to play alone, seem to tune people out, are very active and resistant and have violent tantrums. Adults with autism need assistance and supervision and have trouble relating to other people. According to WebMD, behavioral, intelligence and developmental tests administered by a doctor can determine if an individual is autistic.
Autism Speaks notes a variety of symptoms typically manifested in autistic children and adults, including arranging and rearranging objects, difficulty understanding body language, carrying out monologues during a conversation and difficulty managing their emotions. Repetitive behaviors such as hand-flapping, rocking and repeating sounds are also common. According to the National Institute of Child Health and Human Development, even if an autistic individual has mild symptoms and another has more severe symptoms they both have autism.
Diagnosing autism involves assessing an individual's behavior and testing for physical and medical problems. A physical exam measures weight, height and head circumference to see if the measurements indicate abnormal growth. Doctors examine the individual's behavior by comparing it to diagnostic guidelines developed by the American Association of Childhood and Adolescent Psychiatry and by using developmental and intelligence tests to examine the individual's capacity to make decisions.