How to tell a sheva na from a sheva nach?
Where the leading letter of a word is punctuated with a sheva, it is a sheva na.
Where two letters punctuated with a sheva occur in immediate succession, the second sheva is a sheva na and the first a sheva nach.
Where the occurrence of a letter is immediately followed by another occurrence of itself and is punctuated with a sheva, the sheva is a sheva na.
A sheva punctuating a letter that occurs right after a letter punctuated by a big vowel is, provided that this latter letter is not accentuated (i.e., does not head an accentuated syllable), a sheva na. But if the latter letter is accentuated, then the sheva of this sheva-punctuated letter is a sheva nach.
A sheva punctuating a strongly modulated letter (bearing a dagesh chazak) is a sheva na.
The sheva of the trailing letter of a word is a sheva nach (if two of them occur in succession, both are a sheva nach).
Recall: there are big vowels and small ones. A letter punctuated by a sheva that occurs right after a letter punctuated by a small vowel has a sheva that is a sheva nach. However, this applies only where the small vowel in question has not been truncated. If it has been truncated, then the sheva in question is a sheva na. Furthermore, if the letter bearing a small vowel has received a strong modulation, then the sheva of the ensuing letter is, likewise, a sheva na.
The sheva of a letter that comes right after an occurrence of the vav-of-attachment (vav hachibur) is reckoned a sheva nach (as if it had followed a small vowel).
The sheva of a letter that follows the occurrence of a letter punctuated with a big vowel is, if the letter punctuated with a big vowel is (also) accentuated, a sheva nach.