TT #79 · Parshas Tazria · Aliyah 1 · Passuk 3
English: The priest shall examine the affection on the skin: if hair in the affected patch has turned white and the affection appears to be deeper than the skin, it is a leprous affection on the skin; and if the priest sees that it is a leprous affection, the priest shall pronounce the person impure.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:3
Torah Temimah cites Sifra (Toras Kohanim) for requiring the priest to focus directly on the affliction during examination.
Sifra — Vayikra 13, Midrash Sefer
English: “The lesion”—the priest must keep his eyes on it while examining it.
Textual precision
The priest’s examination must be direct and focused on the lesion itself.