Witches supposedly have "the mark of the devil" on their buttocks, made by a forceful bite of the Devil's teeth, which takes the form of two crossed horns, of a toad, of a ram with large horns, or of a simple little circle.
According to Catalan tradition (again, this may not be uniquely Catalan), one can wash marks on the skin with holy water in order to know whether they are the Devil's work: the Devil's mark will not wash off.