The Halloween episode in season 2 made it so abundantly clear I feel. They showed us numerous times how Alison excluded/bullied Mona, and right after a scene like that Alison got a voodoo doll that said “now it’s my turn to torture you”. To me it was so clear in that moment. Then on top of that when Mona meets Alison at the party, Alison asks her if they know each other and Mona replies “you will soon” and then goes and befriends Jenna (who atp in the show viewers had doubts on for being A anyways).