Charles Ellicott Commentary Leviticus 13:20

"and the priest shall look; and, behold, if the appearance thereof be lower than the skin, and the hair thereof be turned white, then the priest shall pronounce him unclean: it is the plague of leprosy, it hath broken out in the boil." — Leviticus 13:20 (ASV)

Behold, it be in sight lower than the skin. —Better, Behold, its appearance is lower than the other skin. If upon examination the priest finds that the spot has assumed a deeper appearance than the rest of the skin, and the hair turned white—which were the two critical symptoms—he immediately declared it leprosy.