From my observation, I notice that autistics (high and low) tend to be lost in their minds. This is the number one thing I notice. They are either smiling, humming, muttering, making facial grimaces, gesturing--something.
My son will rarely walk anywhere. He moves in a sort of a pattern or shuffles or runs, his body leaning forward, shaking his head back and forth as if he's involved in a video game. He did tell me once that he imagines he is one of the characters. Maybe walking is just too boring for him.
He will also shake his head back and forth and I know he's watching some visual in there, widen his eyes, sometimes mutter something. You have to look closely to notice this.
If he gets an object in his hand that he's wanted, he'll sometimes put it up against his cheek, hold it up at a particular angle, seem enamored by this particular object (whatever it is).
Sometimes, he's humming too.
I'm very observant as you can tell and my son is my life so I'm especially observant of his ways.
btw--my son is extremely high functioning and has a mere dash of autism, but just enough to cause some problems bringing him down to earth at times.
hope this helps.
equinn