Albert Barnes Commentary Leviticus 13:3

"and the priest shall look on the plague in the skin of the flesh: and if the hair in the plague be turned white, and the appearance of the plague be deeper than the skin of his flesh, it is the plague of leprosy; and the priest shall look on him, and pronounce him unclean." — Leviticus 13:3 (ASV)

The hair in the plague is turned white - The sparing growth of very fine whitish hair on leprous spots, in place of the natural hair, appears to have been always regarded as a characteristic symptom.

the plague in sight be deeper than the skin of his flesh - Rather, the stroke appears to be deeper than the epidermis. The bright spot changed to a brownish color with a metallic or oily luster, and with a clearly-defined edge. This symptom, along with the whitish hair, at once decided the case to be one of leprosy.