Baelish did. Well, he didn't come right out and say exactly, but he explained to Sansa that the necklace was a fake, and there was no good reason to give her a necklace with fake crystals on it unless they were poison.
Interesting... then who was it that put the poison in? Sansa seems to be the one who sat the furthest from the King, and with everyone watching it would have been pretty difficult.