DSM (Diagnostic and Statistical Manual of Mental Disorders) Shows clear signs and rules needed to diagnose.
And what it says about autism is:
A. Persistent deficits in social communication and social interaction across multiple contexts, as manifested by the following, currently or by history (examples are illustrative, not exhaustive):
1. Deficits in social-emotional reciprocity...
2. Deficits in nonverbal communicative behaviors used for social interaction...
3. Deficits in developing, maintaining, and understanding relationships, ranging, for example, from difficulties adjusting behavior to suit various social contexts; to (...) absence of interest in peers.
B. Restricted, repetitive patterns of behavior, interests, or activities, as manifested by at least two of the following, currently or by history (examples are illustrative, not exhaustive):
...
4. Insistence on sameness, inflexible adherence to routines, or ritualized patterns of verbal or nonverbal behavior (e.g., extreme distress at small changes, difficulties with transitions, rigid thinking patterns, greeting rituals, need to take same route or eat same food every day).
5. Highly restricted, fixated interests... (e.g., ..., excessively circumscribed or perseverative interests). (He is obsessed with excavators and programming)
6. Hyper- or hyporeactivity to sensory input or unusual interest in sensory aspects of the environment ( he doesn't have much sensory issues but when he was talking about the perfect seat he mention the stable temperature without direct waves from the heather and a/c)
C. Symptoms must be present in the early developmental period (but may not become fully manifest until social demands exceed limited capacities, or may be masked by learned strategies in later life).
D. Symptoms cause clinically significant impairment in social, occupational, or other important areas of current functioning. (When Jae starts destabilizing his life with small acts that disrupt his routine, he can't even handle school, because his stability depends on things that don't affect "normal" people.)
E. These disturbances are not better explained by intellectual disability... (Of course not)
(...) Individuals who have marked deficits in social communication, but whose symptoms do not otherwise meet criteria for autism spectrum disorder, should be evaluated for social (pragmatic) communication disorder.
"Manifestations of the disorder also vary greatly depending on the severity of the autistic condition, developmental level, chronological age, and possibly gender; hence, the term
spectrum. Individuals without cognitive or language impairment may have more subtle
manifestation of deficits (e.g., Criterion A, Criterion B)... "
«Criterion A deficits in social communication will be more subtle if an individual has better
overall communication skills (e.g., is verbally fluent, does not have intellectual impairments).
Similarly, Criterion B deficits (i.e., restricted patterns of behavior and interests) may be less obvious if the interests are closer to age-typical norms (e.g., Ancient Egypt or trains as compared to wiggling a string)»
American Psychiatric Association: Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision. Washington, DC, American Psychiatric Association, 2022.

sangwoo has me fucking dying bruh why is he like thiss