It is important to remember that at the time of the play, there were many beliefs about witchcraft and what the signs of witchcraft were. Betty remains motionless with her eyes closed, and will not move, eat or offer any sign of life apart from lying completely comatose on her bed. When Mr. and Mrs. Putnam visit, they tell Parris that their daughter, Ruth, suffers from exactly the same symptoms:
She ails as she must--she never waked this morning, but her eyes open and she walks, and hears naught, sees naught, and cannot eat. Her soul is taken, surely.
Lying unresponsively and seemingly being unable to communicate and eat were thought of as being sure signs of some form of witchery, and Mrs. Putnam interprets this as the taking of these girls' souls. Note also that in this section of the play Mrs. Putnam asks how high the girls have flown and reports that there have already been rumours that Betty has been seen flying. Such strange symptoms cause her to jump straight away to the conclusion that witchcraft is to blame for what has happened to her daughter and to Betty Parris.