I think it was the Image Comics Free Comic Day issue that Wanda revealed that she knew. Can't remember how she knew though. I kinda figured that when they first crossed paths when the Curse made his apparent and she sought him out (issue 27 I think) laid the seeds for the beginning suspicions.
And Terry knew since issue 54. So who knows what Terry mutters in his sleep, lol. With all the weirdness, Cysn's connection to the 'sad-man', Granny Blake vague wisdom, and Terry, Wanda had plenty of clues.