You can usually tell whether you have nits if your head is itchy especially the warmest parts of your head. ie. behind the ear, neck. Your hair tends to be greasy as well no matter how often you wash it. You will see some white eggs attached to your roots. They look a little like dandruff by whereas dandruff comes out when you shake your head...the eggs will stay firmly in place. You may also be able to see the nits moving about.. if you pull a section of hair up you tend to see them dart to your scalp. Not a pleasent thought, making me itch just thinking about it. I've only had them once when I was in my teens..