How is it determined in Salem that a person is a witch?
Quite often it was enough for a person to have been accused. Once the accusation was made, there was such an enormous amount of doubt and suspicion laid upon that person that it was nearly impossible to escape from it. Forget the idea of "innocent until proven guilty" - these people were guilty until proven innocent, and usually it was impossible for them to prove their innocence.
It would probably be helpful for you to check both the literature link below on the play itself, as well as the information contained in the history section of eNotes on the Salem Witch Trials. Good luck!
Ultimately, as the court scenes prove, it is the reaction of Abigail and the other girls that made the determining factor. Some attempt to "interview" the accused was made by Hale, and even Danforth, but as soon the girls started crying out and announcing that "they feel the presence on them", the accused was assumed guilty and sent off to prison. That is why it is so important for John Proctor to speak out against Abigail, and for him to have Mary Warren back him up. Of course, the plan backfires, and the girls are able to both intimidate Mary and implicate John.