1) One of the most important things we can do as parents is listen seriously to our children. It is awful to be dismissed or made to feel like we are just "imagining things" when the cruelty is all too terrifying.
2) Give each of your children one on one time. This can be hard when we have more than one kid, but you can figure out a way to do it. Find a baby sitter, swap time with grandparents or your spouse.
3) Make that time without agenda. Just "hanging out" in a way that invites conversation provides an environment to share.
4) Trust your gut. If you suspect something is going on, gently ask your kid about it.
5) Do not be tempted to jump in with the solution. Ask your child what ideas they have to stop the bullying; what would they like you to do. Share your thoughts. Suggest a consultation with another adult, the school counselor, for instance. Be sure your child is on board.
6) Empower your child without expecting him or her to "tough it out". As Cruel's Not Cool says: "We want to teach our kids to be assertive, sure, but some social battles have gotten too big for kids to deal with on their own. If you have a hunch something's going on between your kid and a peer please DIG DEEPER and be there to help get to the bottom of it."