"Sie"="you" is always capitalised while "sie"="they" is not. So unless this is the first word of a sentence (or you hear it but do not see it written), you can easily tell.
At the beginning of a sentence though you would have to rely on a context to tell whether it's "you" or "they". Without a context (like in this DL example), you just have to make a guess. Both answers should be accepted by DL though -- please report it if you were marked wrong.