If you really like your girlfriend, don't bring it up. When she is ready, she will tell you what you want to know. You will not like what you hear so be prepared for nausea, if you truly care.
The best way I can tell you something you can relate it to for the "feeling" is imagine yourself naked in town square and all of town is there to see you get your penis branded (because it's the law). You can't NOT do it, everyone is looking and you see and hear the guy in line before you scream. That should give you an IDEA of what sexual harassment and sexual assault are like.
LISTEN from now on though. If it comes up in the future that a female you know is having a bad time with a male in her life, LISTEN. Women are very forgiving of inappropriate behavior so as to not get labeled "prissy" and to be a modern feminist. When it becomes a bad day, that is your cue. Maybe. Tread lightly. Like mentioned, if you push it or force the conversation, you are just as bad as the male causing her a bad day