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.
I will also add- Ells is for when you are talking about a feminine group. Ils can be used for a masculine group or mixed gender group.