Maybe this isn’t much of a give away, but with so much information to take in it certainly avoided me for a while.
There are two instances where this comes up. The first is when Syrio is training Arya for the first time . He notes that after he tells to hold the practice sword in one hand, she goes for the left. The next instance where this comes up that I have found is all the way in “A Feast for Crows”. As she first arrives in Braavos, Arya wears a “…dagger on her left hip and Needle on her right.” Since you keep weapons on the opposite side of the hand you use them in, this is more proof that she is left-handed.
To the best of my knowledge, I don’t know of any other explicitly left-handed characters. Though I would certainly be interested if someone could find them.