Pre-diagnosis
For many autistic individuals, characteristics usually first appear during infancy or childhood and generally follow a steady course without remission (different developmental timelines described in more detail below). Autistic people may be severely impaired in some respects but average, or even superior, in others.

Clinicians consider assessment for ASD when a patient shows:

regular difficulties in social interaction or communication
restricted or repetitive behaviors (often called "stimming")
resistance to changes or restricted interests
These features are typically assessed with the following, when appropriate:

problems in obtaining or sustaining employment or education
difficulties in initiating or sustaining social relationships
connections with mental health or learning disability services
a history of neurodevelopmental conditions (including learning disabilities and ADHD) or mental health conditions.
There are many signs associated with autism; the presentation varies widely:

Common signs for autistic spectrum disorder
avoidance of eye-contact
little or no babbling as an infant
not showing interest in indicated objects
delayed language skills (e.g. having a smaller vocabulary than peers or difficulty expressing themselves in words)
reduced interest in other children or caretakers, possibly with more interest in objects
difficulty playing reciprocal games (e.g. peek-a-boo)
hyper- or hypo-sensitivity to or unusual response to the smell, texture, sound, taste, or appearance of things
resistance to changes in routine
repetitive, limited, or otherwise unusual usage of toys (e.g. lining up toys)
repetition of words or phrases (echolalia)
repetitive motions or movements, including stimming
self-harming