The most obvious sign is a very itchy scalp, caused by the bites where
the head lice suck blood.
If you suspect lice infestation, you should look closely, and inspect the
hair roots close to the scalp. A fine-toothed nit comb can be very helpful
if you want to clarify whether there are lice or not. The comb helps you
discover nits which often might be mistaken for dandruff: whitish,
shimmering nits are empty shrouds from which lice have already come
out, while brownish-grey nits are in their development phase.