And the priest shall look on the plague in the skin of the flesh: and when the hair in the plague is turned white, and the plague in sight be deeper than the skin of his flesh, it is a plague of leprosy: and the priest shall look on him, and pronounce him unclean. (Leviticus 13:3)
He was to examine the infected area, for it might, or might not, be leprosy. There were two signs for which he was to look, white hair in the spot and a depression in the skin—Jews ordinarily have dark or black hair. Where these two conditions were found to exist, he was pronounced unclean.