How can I know if my son is getting bullied?

You can let your son know that he can trust you if anything is happening on his end. It's probably best not to force him to say anything right now if he's not feeling comfortable bringing it up.

Look at your son's behavior. If it appears to be different than usual or odd than something is going on. Kids at a young age tend not to be that open about these things to anyone but their friends. You should probably let your son feel like you are his open-diary and he can trust you like a friend.

Honestly? Asking is the first way to go about it - but when you ask, don't make it sound like an accusatory, or embarrassing thing. You can use stories from your own past to relate (So they feel safer), or you can bring it up directly because of concerns you may have. Most kids (Especially teenagers) hate talking about that, because they feel embarrassed or ashamed to be the victim of bullying, and definitely don't want to talk to mom/dad about it. In these cases; i'd look for signs. Changed behaviors, loss of appetite, bruises, lack of self confidence, a surge of angry emotion, ect. If you're still concerned; maybe consider looking into therapy exercises you two can share?

ask him, but you must have well established relationship with communicating regularly for him to open up and you must be nonjudgmental and accepting of whatever he says. So keep trying and keep listening and keep going and don't give up trying to get through but be willing to listen and don't give advice, be willing to help but ask him how best to help, and if the story involves a surefire threat , make sure he knows you will both need to involve a trusted adult who can take care of the matter with as much discretion as possible

I understand how hard this question is for parents. That's a hard question to ask when we don't know the outcome. Personally the first thing that I noticed with my child is his behavior changed. Him not wanting to go to school was the biggest hint. Once I sat him down to have a heart to heart honest conversation he told me what was going on. It was a really tough conversation but the answer gave me that chance to try and help him through it. Maybe watch his behavior and ask his teachers if they have noticed anything? A heart to heart maybe will give you some answers as well.