This is all very helpful, but how do you spot English?
An understandably naïve view of things whose errancy could easily be corrected through your coöperative perusal of The New Yorker.
The diaeresis indicates that a vowel should be pronounced apart from the letter that precedes it [1]
[1] https://en.wikipedia.org/wiki/Diaeresis_%28diacritic%29#Diae...