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, <em>plays, boys, goes, flies.</em>
This also applies to vowels followed by an 'r', e.g. <em>cars, offers.</em><em> </em>
2. If it's a voiced consonant, 's' is pronounced /z/. For example, <em>needs, flags, pubs, moves.</em>
3. If it's an unvoiced consonant, 's' is pronounced /s/. For example, <em>wants, bricks, cups, cuffs. </em>
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:
http://www.grammar.cl/english/pronunciation-final-s.htm
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!