According to the American Academy of Pediatrics guidelines, the gold standard for diagnosing an active head lice infestation is the identification of a live nymph or adult louse.
These guidelines recommend an examination of wet hair lubricated with such products as a standard hair conditioner. Your doctor will carefully comb your child's hair with a fine-toothed comb (nit comb) from the scalp to the end of the hair. If no live louse is found, he or she will likely repeat the entire exam at a second appointment.
Your doctor will also look for nits in your child's hair. To find nits, he or she may use a specialized light called a Wood's light, which causes nits to appear bluish. But the identification of nits does not necessarily confirm the diagnosis of an active infestation.
A live nit needs to be near the scalp to incubate. Nits found more than about 1/4 inch (6 millimeters) from the scalp are likely dead or empty. Suspect nits can be examined under a microscope to determine if they're living — evidence of a likely active infestation.
If no live nits are found, they're probably left from a previous infestation and not evidence of an active infestation.