To wolves, channelers are "marked" and are said to stand out from the rest of their pack. I'm not sure if this ability is also held by Wolfbrothers, but given that wolves often warn Perrin about threats such as these in the dream, it wouldn't take much to reveal Aran'gar as an agent of the Shadow and a channeler.

Also Aran'gar is so evil that Perrin would start to pick up on this threat of violence as soon as he comes around.