Speaking only from my personal experience of one person's very specific journey of self-discovery, one of the hazy clues about me being autistic was my tone of voice and eye contact.
While I meet allistic people's gaze an appropriate amount, and usually inflect my voice with the appropriate intonations, there's something important to bear in mind: while I can do these things, I'm doing them because I consciously learned to do them at a very young age. So young, in fact, that I don't even remember consciously learning to do them, but I did.
Every now and then I'll "check in" with myself, to make sure I'm maintaining an acceptable amount of eye contact, not too much, not too little. Whenever I talk about something I'm passionate about, my speech will get fast, monotonous, and slurred, as I'll forget to keep up the intonations expected of me. When I try to pay someone a compliment, they'll often interpret me as insulting them, so clearly my tone of voice leaves a lot to be desired anyway.
If you're autistic, it's not necessarily like you can't learn to read and emulate facial expressions and body language (some people can, some can't), it's just that you need to consciously learn to do it, like riding a bike. That's apparently not how allistic people learn to do it. They just naturally learn to talk that way, without having to make an effort to do so. I think this is an important distinction.
This is one of those things where being able to perform it seemingly means you're not autistic, but the fact you have to consciously perform it (no matter how used to it you are by now) means you are autistic. It's not how good you are at emulating body language that suggests whether you're autistic or not. It's the fact you're emulating it at all.
And then I suddenly remembered that period of ninth grade where I got really into psychology... specifically how to read people's expressions and body language for nonverbal cues. This was an extracurricular project, I was not being told to do it, I just knew that I was shit and wanted to get better. So I studied that shit. I literally took notes lmao and regularly took online quizzes. And I remember how conscious of an effort I had to make to scan people's faces and bodies and try to remember what that foot posture meant, what it might mean if they're glancing a certain direction, if they're fidgeting or quirking their eyebrows a certain way. If that sounds exhausting it's because it was, and kind of still is. If I'm around someone enough times, I won't have to think so consciously about all these shenanigans. If it's someone new, it's like I'm back to 100% manually assessing this information.
— galaxybutt, 2021