How do you identify a well spoken or written english sentence?

Is it because of our knowledge of the rules of English language?( of which am sure 60% of us are not conversant with, I stand to be corrected though.)

or is it because it has been used by people who appear to be conversant with the rules of English language?

Or because the grammar sounds correct?

I ask these questions because Nigerians are quick to laugh at grammatical blunders.