I think my girlfriend is faking her orgasms. How can I make sure that she cums?

Well, first you should talk to her. There is no magic formula to make a woman cum. Every woman is different. So, if you think she is faking it, you're probably right. Yet, you won't know for sure unless you talk to her.

Being in a relationship, you have to communicate. You can approach the subject by saying something like, "It's not going to hurt my feelings. I'm not going to get upset about it. I'm not going to be sensitive about it. I just want to make sure that you cum. It turns me on. What can I do for you to make you feel good and to make sure that this happens?"

Communication is the first step. Past that, you can also try toys...but the most important thing you can do is talk to her to find out what she likes.