Do you have an internal stream of language and ideas? Can a lack of internal dialog lead to lack of (self) awareness? Is it required for the proper application of logic? Fundamental questions I think.
People Are Shocked to Discover That Not Everyone Has an Inner Monologue
Worthy of consideration. How to identify?
Maybe something more subtle, before we get there.
I'm not sure it's a "quiet mind" so much as a different way of processing information.