Look for nits. The nits are egg-cases, attached to the hairs. They are oval in shape. The empty egg cases (left behind after the baby louse has hatched) are white, and easier to spot. As the hair grows, the egg case will move further up its shaft, so the position of the nits on the hair gives you an idea of how long you have had them. Hair grows at about 1 cm a month. Therefore nits 1 cm from the scalp mean 1 month, 2 cm mean 2 months, and so on.
If you have never seen them before, it can be difficult to decide whether a white speck is a nit or not. Globules of hair lacquer and bits of dandruff can be confused with nits. And if you pull out a normal hair, you may notice the whitish thickening at its root, which you may mistake for a nit. A magnifying glass will help.
Look for black specks on the pillow or collar, these are louse faeces.
Look for tiny, inflamed bites on the scalp, or a rash on the back of the neck or behind the ears.