Jamieson, Fausset and Brown on Leviticus 13:40-41

40-41. bald ... forehead bald--The falling off of the hair, when the baldness commences in the back part of the head, is another symptom which creates a suspicion of leprosy. But it was not of itself a decisive sign unless taken in connection with other tokens, such as a "sore of a reddish white color" [Le 13:43]. The Hebrews as well as other Orientals were accustomed to distinguish between the forehead baldness, which might be natural, and that baldness which might be the consequence of disease.