well topcat1 in my experience, the evidence is immediate, scratching and such like (god youve got me scratchin now lol) my daughter had them loads of times at primary school (it never seems to happen now shes at high school) but if she has dark hair you will see little white eggs that stick to the hair at various places, buy the special comb from the chemists, and section all the hair off, (tedious job) and thn keep coming through, have tissues handy and then if there are any eggs or they have hatched into headlice they will be moving about, not sure how obvious they are on blonde hair,but i'm sure someone will be able to tell you on here.
This is only my experience though, there are also combs that you can buy now that 'bleep' when they detect nits in the hair, and i think they are quite pricey?
good luck