Give her the witches swimming test, as she IS a witch
As part of the infamous “swimming test,” accused witches were dragged to the nearest body of water, stripped to their undergarments, bound and then tossed in to to see if they would sink or float.
Since witches were believed to have spurned the sacrament of baptism, it was thought that the water would reject their body and prevent them from submerging.
According to this logic, an innocent person would sink like a stone, but a witch would simply bob on the surface.