If you want to know if she is faking it, try some experiments. Some easy ways to gauge her honesty is to do a thorough investigation. Whatever you do, don't tell her what you're doing. If she is faking, this gives her an unfair heads-up about your intentions.
First off, be prepared for the truth. Then be prepared to do something about it if there is a problem. Have a frank and honest discussion with her about your sex life in a nonjudgmental way; she knows what she is doing and if she is a faker, she's no doubt duly ashamed of herself. She probably won't admit that she lied, but you can ignore that and just start working on how to give her some real orgasms. Don't let her fool you anymore -- do something constructive about it. It has been proven time and time again that partners who regularly communicate openly and honestly have the best sex. Don't be the couple who loses out.
Ask her what the sex/stimulation feels like, and ask her to explain what happens to her body when she is having an orgasm. Use normal lie-detecting techniques (body language signals) to see if she is lying. Touching her mouth (covering her lies) and looking away as she speaks to you are some ways to tell. Obviously, these are not set in concrete, but if you ask her directly if she orgasms when you have sex and she looks away and says "Yes, of course, honey!" then further investigation is in order.