When I ask parents when they first noticed their child's head lice, it usually becomes clear that the signs of head lice were present long before the parents would like to admit. They say things like, "Sure, my daughter's head was really itchy, but I thought it was just because of dandruff" or "My son's been taking swimming lessons and I thought his head was itchy from the chlorine". Sometimes, these parents have had no experience with head lice or they never knew anyone who had it so the possibility of head lice doesn't even come to mind. However, many parents are just in denial about head lice because they don't want to have to deal with it. Later they tell me, "I thought it might be head lice but I didn't do anything at first because I was too busy" or "I knew that her friend at school had recently had head lice but I didn't think it would happen to my child".
In my house we have a rule - if you scratch your head, you get a quick spot check. It is even true for me - if I scratch my head, my youngest daughter says, "Mom, c'mere. Let me take a look." And these spot checks, because they are not always reliable, are still done in conjunction with weekly wet combing checks. Of course, someone's head is really, really itchy, they get a wet combing check right away. (I should mention that routine combings will often prevent the itchiness of head lice. Most people are asymptomatic - they don't get an itch - until they've had head lice for quite a while. Routine combings will often find the bugs before the itching starts. It happened to me!)
If someone is itchy, investigate. If you find that you or your child do have a lot of dandruff, use a dandruff shampoo to remove the dandruff before doing a check for head lice. With head lice, you need time to be on your side and the sooner you find the little critters, the sooner you can get rid of them.