A good measure is to comb daily with a narrow toothed comb. Vigorously comb hair over a white towel or the sink or bath regularly, which will show you if there are lice there - they are difficult to spot. Look around the ears and hairlines. If you damage them by combing they can't breed. Also get pictures of the eggs and the empty shells - lots of people think their child still has them when they are looking at the empty shells, which stick to the hair and so grow away from the scalp. Live eggs will be very close to the scalp.
In the UK they used to sell a comb that somehow electrocutes the lice, it beeps when it hits one. I knew several parents who used them there, but have never heard of them out here. Maybe an internet search would find it, if anyone does, let me know!
When you work with young children you do realise how frequently they get them, and it ceases to seem so horrible. Girls get them more often because of long hair and because they often touch more, so keep long hair tied back.
The other thread was on Healthy home, I think.