I have two questions.
First, how do I know that the "S" at the end of a noun pronounced /s/ or /z/?
Lastly, how do I know that the "S" at the end of a verb pronounced /s/ or /z/?
The answer to your two questions is the same - the pronunciation of the final 's' depends on whether the sound that proceeds it is a vowel or a consonant sound. If it's a consonant, it depends whether it's voiced or unvoiced.
1. If it's a vowel, 's' is pronounced /z/. For example, plays, boys, goes, flies.
This also applies to vowels followed by an 'r', e.g. cars, offers.
2. If it's a voiced consonant, 's' is pronounced /z/. For example, needs, flags, pubs, moves.
3. If it's an unvoiced consonant, 's' is pronounced /s/. For example, wants, bricks, cups, cuffs.
That is a great question and I could not think of a way to explain it, so I looked it up. Here's what I found after a Google search:
It's a complicated explanation, but there is a chart at the bottom of the page that might help. Good luck!
Thank you so much!