More often than not, the subjects reveal their diagnoses themselves at a later point.
If you have guessed correctly from before the reveal for many different individuals, you can pat yourself on the back.
For those that don't reveal or might not even have a diagnosis or be aware they could be, a pile up of additional (unrelated to the initial impression) diagnostically consistent behaviors and mannerisms as you get to know them over time is also a good enough confirmation for use outside of a clinical setting.
In general, even the average non-trained or unfamiliar with the specific traits neurotypical person is good at this identification even subconsciously, they just don't know what exactly they're identifying (so pit it as "weird", "offputting" etc):
(...) across three studies, we find that first impressions of individuals with ASD made from thin slices of real-world social behavior by typically-developing observers are not only far less favorable across a range of trait judgments compared to controls, but also are associated with reduced intentions to pursue social interaction. These patterns are remarkably robust, occur within seconds, do not change with increased exposure, and persist across both child and adult age groups
"… a static image was sufficient for generating negative first impressions of those with ASD. (...) In contrast, first impressions of TD [typically developing] controls improved with the addition of a visual information, suggesting that unlike the ASD group, visual cues helped rather than hurt the impressions they made on observers."
If you have guessed correctly from before the reveal for many different individuals, you can pat yourself on the back.
For those that don't reveal or might not even have a diagnosis or be aware they could be, a pile up of additional (unrelated to the initial impression) diagnostically consistent behaviors and mannerisms as you get to know them over time is also a good enough confirmation for use outside of a clinical setting.
In general, even the average non-trained or unfamiliar with the specific traits neurotypical person is good at this identification even subconsciously, they just don't know what exactly they're identifying (so pit it as "weird", "offputting" etc):
(...) across three studies, we find that first impressions of individuals with ASD made from thin slices of real-world social behavior by typically-developing observers are not only far less favorable across a range of trait judgments compared to controls, but also are associated with reduced intentions to pursue social interaction. These patterns are remarkably robust, occur within seconds, do not change with increased exposure, and persist across both child and adult age groups
https://pubmed.ncbi.nlm.nih.gov/28145411/
Even photographs will do it:
"… a static image was sufficient for generating negative first impressions of those with ASD. (...) In contrast, first impressions of TD [typically developing] controls improved with the addition of a visual information, suggesting that unlike the ASD group, visual cues helped rather than hurt the impressions they made on observers."