That is a great question. Here are some clues.
If the word has two "ee"s as in feed it will say the name of the letter. Usually when two vowels are together in the middle of a word they say the name of the first vowel, as in "peak".
Also, if there is an 'e' at the end of the word, it is silent, but it will make the vowel in the middle say the name. If there is not an "e" at the end of the word, the vowel in the middle will say the sound, as in "fed".