Yes, there´s a rule.
After verbs that end in ch, ss, sh, x, g, we add "es" and the pronunciation is /iz/.
e.g. wash-washes; pass-passes; fix-fixes; watch-watches.
The same happens with nouns in plural.
beach-beaches; bridge-bridges; party-parties; language-languages /iz/
Verbs in 3rd person that end in t, k, p, f, add "s" and the pronunciation is /s/
think-thinks; help-helps; get-gets; laugh-laughs.
In all other cases the final "s" is pronounced /z/.
e.g. play-plays; love-loves; friend-friends.
Hope that helps.
Student or Learner