Listen to the Accuser
If someone is accusing your child of bullying, listen carefully to what is being said. If you have the wherewithal to take out a notebook and jot down notes, do so. This is traumatic news and you may forget later on what was said about your child's behavior.
Detailed information from the perspective of the perceived victim arms you for a more effective talk with your child. Knowing all the details of what is occurring also provides a teaching opportunity: first, about how the child can change these behaviors, and second, but no less important, how awful someone else feels when bullied.