When an anonymous mom asked what are the signs of child molestation (sexual abuse), it got me thinking about all the women I know who were molested as girls. All the men I know who were molested as boys. Child molestation is so insidious; its victims are affected for life--and often carry their "secret" with them throughout adulthood. Talk to your children about the reality--every adult is not to be trusted, and if any adult ever makes them feel uncomfortable or asks them to do something they don't want to--they should say "no!" and tell you immediately.
Here is a list of some of the most common signs of child molestation (of course, individually they may not point to that, but in all cases trust your instinct) :
If a child tells you that they have been sexually abused in any way, believe them. Children rarely lie about sexual abuse and 80% of all child molestation cases involve someone the child knows.
It is NEVER a child's fault.
If you have been a victim of sexual abuse yourself, it might be helpful to join one of the private groups on CM like Overcoming Sexual Abuse (which amazingly has nearly 600 members).