How exactly do we know they dont have reflective thought? Or at least any less reflective thought than your average human. Not really sure how to test that. I k

How exactly do we know they dont have reflective thought? Or at least any less reflective thought than your average human. Not really sure how to test that. I know there is the mirror test but what if there are different types of reflective thought and they just missed the boat on that by a little but have other kinds of thoughts?