The easiest way to know you have lice is that you itch. This comes from an allergic reaction to the saliva of the insect injected under the skin when it feeds. Unfortunately only 50% of us are allergic to the saliva and thus only about 50% of us itch. (Drat!)
The second best way to know you have an issue is by seeing them on the head. Lice hate light and need the warmth of the head to lay eggs. Checking under the bangs on the forehead, in the hair just above and behind the ears and in the hair near the base of the neck are the easiest places to see them. Parting the hair quickly across the back of the head helps you to catch them as they flee from the light. In warmer climates, like in Texas, they tend to move to the top of the head. Every person’s infestation is different.
Finally, running a good quality lice comb across the head from front to back will help make clear what every mother dreads –LICE!