I suppose the best information I can give you around this topic is to listen to what your child is not saying. What do I mean by this, I mean notice if your child is acting differently, specking differently about themselves.
I say this as a person that was very badly bullied for a long time. I never told anyone because I was afraid that I would get in trouble. I became the nice boy who never got into trouble and did what he was told at home. By doing this I never drew attention to myself or what might be happening me at school. This is what I mean when I say listen to what your child is not saying.
For me I know I would have liked to have been asked questions around my behaviour for example,
How is school going for you Mike?
Has anything changed for you in school?
I am here for you if you would like to talk.
As you can see all the questions are about how the child is feeling. I find if you talk about the child and only the child you can in turn make a safe space for the child. If you start asking about other kids and if these kids are friends or not with the child, it take the spot light off the kid and they are less likely to open up. The child is looking for a safe place to just be themselves.
The relationship with yourself is the most important.