There's a YouTube channel called, "I'm Autistic, now what?" And one of her videos mentioned this. She said what differentiated autism from introversion for her was she constantly paid attention to her facial expressions and mannerisms while talking with others. But her introverted family members didn't do that. They just aren't big into conversations.

That's not to say that's the case for everyone, just what she herself noticed between herself and her relatives.

@cogitoprinciple I only like events like that if I know I‘ll have an interesting conversation partner who will stick with me the whole time. Nothing more awkward than finding myself alone having to acquire one. If I’m successful, it’s not even a permanent solution because they like changing their partners at these occasions 😂. And the stuff that‘s being talked about usually is of no interest whatsoever. Listening to my autistic self and not going saves me a lot of hustle now.