Assessments are based on a combination of evaluations which usually includes reports from parents, health professionals (including psychologists, paediatricians etc) and schools. Assessment will also consider current and previous developmental history. The DSM-5 breaks down the signs and symptoms of ASD into categories. It also states how many of these must be present in each category to confirm a diagnosis of ASD and where on the spectrum they fall .
Professionals who can diagnose autism spectrum disorder:
You will need a referral to see these professionals, so your GP or Paediatrician is a huge resource in assisting you co-ordinate these services. Autism NT can provide information on current practicing clinicians who have an interest in ASD. The Raising Children Network has a lot of information on how to obtain a diagnosis with interactive Autism Services Pathfinder diagrams: raisingchildren.net.au/services_pathfinder/services_pathfinder.html
Do not be afraid to seek a second opinion!