You won’t get the information from the pronunciation…

Bon… certains font néanmoins des efforts… comme les moutonsssses du très fameux Topaze. 😉

For a verb, only the subject can tell you. There should always be one.

Of course if you cannot get the information from the subject as in Il mange or ils mangent, then only the context, telling to what the pronoun refers, will tell you.

For a noun, only the article can immediately tell you. (*)

Of course, if you cannot find the article as in adieu veau, vache, cochon, couvée then only the context can tell you.

If the context does not, then… you are free to write your own understanding regarding the number of elements you would involve in such a situation.

(*) In some very rare cases some irregular adjective could also tell you.
appartement royal. royal => singular for appartement / appartements royaux. royaux => plural for appartements.