Phonetically speaking, you can’t tell the difference between them; they are pronounced the same. And yes, it goes for all the other verbs where the third person singular is pronounced the same as the third person plural, except in cases where the verb starts with a vowel. In those cases, there is often a liaison made when it’s plural. For example, ils écoutent will be pronounced “il (z)écoute” as opposed to “il écoute” for singular.

So how do we ever tell the difference between the two? Context. It is usually fairly clear from context if it’s singular or plural. There are also often other words in a sentence that will clear up the meaning if the subject is muddy (e.g. are we talking about the group or just one individual). Such words can be tous, ensemble, seul, etc.

Usually, we rely on the context to determine whether the pronoun is the singular “il”/”elle” or the plural “ils”/”elles”.

For example :

Ils sont partis. (They are gone.)

The auxiliary “sont” is plural, so the pronoun before it is plural as well.

While in :

Il est parti. (He is gone.)

The auxiliary “est” is singular, so the pronoun before it is singular as well.

For verbs like “parler” (to speak) that don’t change phonetically between singular and plural (“il parle” and “ils parlent”), the number must be implicit.

Just seeing this question 3 years later, but I have noticed that in DuoLingo, and there isn’t always context from which you can determine singular or plural form. But, I’ve also noticed in pronunciation that there is a slightly prolonged, more pronounced release of the final syllable in the plural form. So instead of “elles” sounding short like “elle”, it will sound more more like “el-uh”. “Écrivent” instead of sounding like “ècrive” will sound more like “écriv-ffe”. It is ever so slight. You can hear it better when you listen to the slow version. Truth is, the average person speaking casually in French will probably not make such distinction, so unfortunately, you’ll have to rely on context in most situations, as verbs like “être”—which make the form obvious—are not always used.

That slight distinction actually does make sense, and if I were to speak it, I would probably pronounce the words that slightly differently, with the extra emphasis at the end. I suppose it is like saying the difference between “Legal” and “Eagle” in English. The space between my tongue and the roof of my mouth is slightly raised in “Legal” than in “Eagle.”