When your child is invited to spend time or have a sleepover at a new friend's house, find out the names of all the adults who live in the house. Then take the time to look for each of them on the National Sex Offender's Registry http://www.nsopr.gov/.
Remember that only a small percentage of child molesters are caught, convicted and required to register. So do not let the registry provide a false sense of security that you know where the danger lies.
I was speaking with a very diligent father about the epidemic and he shared that his daughter had been invited to a sleepover at a friend's house. When he found out her parents were divorced and the sleepover was at the Dad's house, he took the time to look on the National Sex Offender's Registry and indeed found that her father had been convicted of child sexual abuse.