The main piece of evidence that is used to back up Abigail Williams's claim that Elizabeth Proctor is a witch is the "poppet" that is found in Proctor's home. This is a doll that Proctor has allegedly been using like a voodoo doll.
Abby says that Proctor has used the doll to bewitch her. She screams with pain and says that Proctor has done something to make her stomach hurt. When the men go and find the poppet and Proctor's home (it was put there by Mary Warren) they find it has a needle stuck in its stomach. This is the proof they need to arrest Proctor.