I actually did not observe that. I inferred it from the dialogues. However, apparently the book says otherwise. Then again the series always been a bit different from the books.
Otherwise ? how?
IIRC, the same is in the books....
....told by Littlefinger to Sansa, that Olenna Redwyne (that 'buddhi' Tyrell) murdered Joffrey in the same way - taking a bead from Sansa's necklace (which I suppose was presented by herself, unlike as shown it was given to Sansa by Ser Dontos).