Leviticus 13:20

And if, when the priest seeth it, behold, it be in sight lower than the skin, and the hair thereof be turned white; the priest shall pronounce him unclean: it is a plague of leprosy broken out of the boil.

Cross-reference

Leviticus 13:3 establishes the exact criteria (hair turned white, deeper than skin) repeated in verse 20 for a boil.

Leviticus 14:37 extends priestly inspection to houses, using similar criteria for spreading plagues.