how can I tell if Elle means 'they' or 'she'?
"Elles" is they. "Elle" is she. They sound the same, but the other words in the sentence (especially the verb) will give you a clue as to which one it is.
e.g. "Elles sont" for they are, but "Elle est" for she is.
It's the same with il/ils.
That's better than I did explaining that!
Depends on the rest of the sentence I struggle with this too and I'm not good at explaining so I'll let someone else but I hope this helps a little...
See the verb to know if it's plural or singular.