If King has Aria pinned as "A," other seemingly harmless coincidences could be clues. Like Aria's name, and the fact that she's the one saying, "Sh," in the intro.
Remember that time Mona said, "Miss Aria, you're a killer, not Ezra's wife"? Even though the girls figured out it was an acronym, where the first letters of the sentence spelled, "MAYA KNEW," the original sentence itself is an interesting choice.
Why It's Not Possible:
In the Halloween episode in season three, Aria gets trapped in the box with Garrett's corpse, and she almost falls to her death off the train. Pretty strange thing to happen to "A."
"A" filled Ella Montgomery's car with bees. Seems like Aria would have to be pretty committed to her master plan to put her own mother in danger.
If she was, indeed, "A," she wouldn't get texts when the other girls weren't around and wouldn't expose her relationship with Ezra.