Ideally the diagnosis of ASD should be given by a team of clinicians (e.g. pediatricians, child psychiatrists, child neurologists) based on information provided from the affected individual, caregivers, other medical professionals and from direct observation. Evaluation of a child or adult for autism spectrum disorder typically starts with a pediatrician or primary care physician taking a developmental history and performing a physical exam. If warranted, the physician may refer the individual to an ASD specialist who will observe and assess cognitive, communication, family, and other factors using standardized tools, and taking into account any associated medical conditions. A pediatric neuropsychologist is often asked to assess behavior and cognitive skills, both to aid diagnosis and to help recommend educational interventions. Further workup may be performed after someone is diagnosed with ASD. This may include a clinical genetics evaluation particularly when other symptoms already suggest a genetic cause. Although up to 40% of ASD cases may be linked to genetic causes, it is not currently recommended to perform complete genetic testing on every individual who is diagnosed with ASD. Consensus guidelines for genetic testing in patients with ASD in the US and UK are limited to high-resolution chromosome and fragile X testing. Metabolic and neuroimaging tests are also not routinely performed for diagnosis of ASD.

The age at which ASD is diagnosed varies. Sometimes ASD can be diagnosed as early as 18 months, however, diagnosis of ASD before the age of two years may not be reliable. Diagnosis becomes increasingly stable over the first three years of life. For example, a one-year-old who meets diagnostic criteria for ASD is less likely than a three-year-old to continue to do so a few years later. Additionally, age of diagnosis may depend on the severity of ASD, with more severe forms of ASD more likely to be diagnosed at an earlier age. Issues with access to healthcare such as cost of appointments or delays in making appointments often lead to delays in the diagnosis of ASD. In the UK the National Autism Plan for Children recommends at most 30 weeks from first concern to completed diagnosis and assessment, though few cases are handled that quickly in practice. Lack of access to appropriate medical care, broadening diagnostic criteria and increased awareness surrounding ASD in recent years has resulted in an increased number of individuals receiving a diagnosis of ASD as adults. Diagnosis of ASD in adults poses unique challenges because it still relies on an accurate developmental history and because autistic adults sometimes learn coping strategies (known as 'camouflaging') which may make it more difficult to obtain a diagnosis.