Is there a difference between second language learners who have internalised the language and native speakers?
Can you ever become so fluent that the language becomes a part of the mental grammar in your head, almost as if you had learned it when you were a language-sucking monster (i.e. a child)? Because I suspect my friends have the same 'intuitions' about English as I do, and I trust their judgements on grammaticality.
As a mostly monolingual, I don't know what it feels like to be that proficient. Is that your goal as a language learner
when you start out learning a language? Have you met such people? Are you one yourself?