3x23
Wren tells Spencer that Alison got CeCe kicked out of UPenn
4x10
CeCe's old roommate tells Aria that CeCe blamed Alison and the Liars for her being kicked out of UPenn
These were hints dropped along the way. Notice how they haven't touched on these yet, but instead portray CeCe as an ally to Alison?
5x01
CeCe shows up in New York and acts friendly with Alison. The two hug before CeCe departs to Paris. Alison then goes on to tell Emily that part of the reason CeCe killed Wilden was because of her.
5x12
CeCe gives Alison a Christmas present, and subtly informs her that Hanna and Spencer were going to investigate her house. CeCe, doing a favor, scares them away as Alison looks on from behind the shelf full of masks.
You can also read in between the lines in the Christmas scene that Alison tells CeCe everything, which is how A stays one step ahead.
Quote:
Originally posted by waryofothers
Almost as perfect as Queen C.
"Mona played with dolls. I play with body parts." Think about it.

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Now it all makes sense!