Typically diagnosis is made after an interview (about an hour) and questionnaires. If there's some doubt about the diagnosis you may need to bring in old school reports. It's also possible to diagnose using brain scans now (e.g http://www.sydneydevelopmentalclinic.com.au/brain_scanning.h...), and medication can be trialled and selected on the basis of an individual response to the medication (shown with another brain scan, or using testing, e.g http://www.sydneydevelopmentalclinic.com.au/medication_testi...).