The "poppet" as it is called, is found in Proctor's home with a long needle embedded in it. Mary had given the doll to Elizabeth, having made it while waiting in court. The "gift" is odd and suspicious but Elizabeth accepts it.
Later, there is a warrant issued to search the Proctor home for evidence of these very dolls that are now deemed to be a tool of witchcraft. John hands over the doll to Ezekial and the large needle is soon discovered embedded within it.
At the same moment that Elizabeth is handling the doll, Abigail is clutching her stomach and produces a two inch needle from the folds of her dress (but makes it look like she's pulled it from her stomach).
For the court, this is ample evidence that Elizabeth is indeed a witch and the poppet one of her evil tools.