There were never any real signs of witchcraft being practiced in Salem, except of course, the girls dancing in the woods with Tituba. As Abigail says, 'It were sport!' None of the other accused citizens had ever been caught in the act of performing anything which could even be slightly deemed demonic, and so too with John and Elizabeth Proctor.
The only reason they became suspects is the fact that Abigail had, in a fit of vengeance, decided to punish both of them - John for rejecting her and Elizabeth for dismissing her. The fact that John had chosen to be with Elizabeth made Abigail see her as an obstacle preventing her from being with him herself. Getting rid of Elizabeth would have made John available.
The girls were terrified of Abigail and continued the charade at her behest. Others were encouraged into making their accusations for ulterior motives.
It is ironic that the progenitor of all wickedness in the town, Abigail, was never accused and that many good people died because of her evil.
First of all, there is no empirical evidence of any witches at all. Every accusation is from other people, so-called witnesses, and essentially just people pointing fingers. At the beginning of the play, Betty's sickness is attributed to the girls' involvement with witchcraft, but it could easily be attributed to a virus or simply the stress of being caught dancing in the woods at night.
In court (in Act Three), Mary Warren tells Danforth that Abigail has been lying about everything. When he questions Abigail, she claims that Mary has changed into a sort of ghost and is attacking her. There is no evidence other than what Abigail is saying. So, Abby is either telling the truth or lying. Considering that Mary is still there, shouting to stop making these false claims, the court is left to believe either Mary or Abby. However, after being accused by more than just Abby, Mary gets scared and also withdraws from supporting John Proctor. She even gets to the point of saying to him, "You're the Devil's man!" Shortly after, she says to Abby, "I'll never hurt you more!" Mary is so distraught, she just wants to be done with the whole thing. She was badgered and coerced into these claims. A good attorney would have exposed this in court and then the judging parties (or judge) would have to look more skeptically all of the accusations. So, even when someone (such as Mary, a scared impressionable girl) seemingly admits to anything, such an admission is done under duress and extreme social pressure. In this sense, Mary felt forced (by the adults and her peers) to turn on Proctor.
In each case, there is no physical evidence of witchcraft. The only evidence is what someone said. In addition, we should be skeptical of Abigail's claims about John and Elizabeth because she (Abigail) has a motive for wanting to hurt them or get them into trouble. John and Abby had been having an affair. John, realizing the error of his ways, ends the affair. Abby wants it to continue. When John refuses, she begins to exact her revenge. This fact alone (Abby's motive) should raise real doubts about Abby's accusations.
Going along with discrediting Mary Warren:
The interrogator should be able to get her to admit to having given the poppet to Elizabeth, thus establishing some collusion of Elizabeth with Abigail who later claims to have been stuck with a pin, a pin akin to the one that Elizabeth Proctor has in the doll given to her by Mary.