> If the gap were so wide it seems like there would be more obvious giveaways in human behavior.

There was a couple who streamed on youtube who reacted to meme compilations, and years ago one of the things they reacted to referenced a similar topic. During that stream everyone learned the woman has no inner voice (including herself, she'd never even thought about it before and had no idea an inner voice was a thing).

While he was asking her what it was like, he brought up the voiceovers in tv and movies that were supposed to represent a character's thoughts. She had no idea that's what it was supposed to be, and always just thought it was some nebulous narrator describing what was going on.

I think any such signals would be a similar "wrong mental model" situation where you can usually only tell by poking at it with very specific questions or actions.