If you go for an autism assessment, the clinician will use a diagnostic tool to help decide whether you are autistic or not.
These tools include the DISCO (Diagnostic Interview for Social and Communication Disorders), the ADI-R (Autism Diagnostic Interview - Revised), the ADOS (Autism Diagnostic Observation Schedule) and 3Di (Developmental, Dimensional and Diagnostic Interview).
They all assess you against a set of criteria for autism, found in diagnostic manuals ICD-10 and The DSM-5.
This page gives you an overview. For more information about the manuals and criteria for an autism diagnosis, select from the menu above or the guide link below.