Here's how you can know.
They will either tell you directly or show you through how they act.
1.She compares you to other guys
If you do something wrong to her, she immediately tells you how 'the guy she told you not to worry about' does that for her girlfriend and she wishes you'd pick a page from his book.
2.She doesn't bother communicating with you
She rarely replies to your texts and calls because she's doesn't care what you have to tell her anymore. And when she does eventually respond to you, she won't be as excited to be speaking with you.
3.She doesn't show any empathy towards you
If she completely disregards your feelings then it is possible that she doesn't care for you anymore.
4.It's all about them
You are noticeably not in the picture anymore. All the decisions they make is about them and less about the two of you.
5.They never make time for you
When someone cares about you they will make a point of creating some time for the two of you to be together.