From listening? You can't. They sound the same. Perhaps if you had read some of the other comments first you would not be so confused. Usually there is some context around the sentence in actual speech that would tell you whether it was one person or several. Which is why this particular sentence if you are listening to it accepts both Ils mangent and Il mange as correct. It wouldn't accept Il mangent or Ils mange however as those are grammatically incorrect.
Ariaflame: I just repeated this lesson to try and improve and once again it insists on the plural translation for "Ils mangent du pain." A lot of the confusion and frustration for students could be avoided if the printed question included a simple (s) for singular and (p) for plural.
Sorry, I should have made that clear. I meant that "Ils mangent du pain." was the spoken French one had to translate, naturally it could not be distinguished from "il mange du pain". It would not accept the answer as singular.
My suggestion is that the question "Translate this text should include a (s) or a (p) to indicate what is required. ie: Translate this text (p)