Accurately diagnosing autism spectrum disorders in adulthood can be challenging. Structured questionnaires and diagnostic measures are frequently used to assist case recognition and diagnosis. This study reviewed research evidence on structured questionnaires and diagnostic measures published since the National Institute for Health and Care Excellence evidence update. The Cochrane library, Medline, Embase and PsycINFO were searched. In all, 20 studies met inclusion criteria. Sensitivity and specificity of structured questionnaires were best for individuals with previously confirmed autism spectrum disorder diagnoses and reduced in participants referred for diagnostic assessments, with discrimination of autism spectrum disorder from mental health conditions especially limited. For adults with intellectual disability, diagnostic accuracy increased when a combination of structured questionnaires were used. Evidence suggests some utility of diagnostic measures in identifying autism spectrum disorder among clinic referrals, although specificity for diagnosis was relatively low. In mental health settings, the use of a single structured questionnaire is unlikely to accurately identify adults without autism spectrum disorder or differentiate autism spectrum disorder from mental health conditions. This is important as adults seeking an autism spectrum disorder diagnostic assessment are likely to have co-existing mental health conditions. Robust autism spectrum disorder assessment tools specifically for use in adult diagnostic health services in the presence of co-occurring mental health and neurodevelopmental disorders are a research priority.