How is a head lice infestation diagnosed?

The diagnosis of a head lice infestation is best made by finding a live nymph or adult louse on the scalp or hair of a person. Because nymphs and adult lice are very small, move quickly, and avoid light, they can be difficult to find. Use of a magnifying lens and a fine-toothed comb may be helpful to find live lice. If crawling lice are not seen, finding nits firmly attached within a 1/4 inch of the base of the hair shafts strongly suggests, but does not confirm, that a person is infested and should be treated. Nits that are attached more than 1/4 inch from the base of the hair shaft are almost always dead or already hatched.