Many people lie, including girlfriends. If you think your girlfriend may be lying it would be a good idea to talk to her about it. Trying to catch her in a lie could backfire on you.
2 Additional Answers
If your girlfriend is lying to you, eventually you will find out. No one gets away with it forever. We often know when it is happening even though we may not want to admit it. Go with your gut feeling. You could always try attempting some type of set up. You could even ask a question that applies to her that you already know the answer too and see what kind of response you get. See if she tells you the truth or not. There are also body languages and facial expressions that will also reveal that she may be lying to you. You can find more information here: www.blifaloo.com
To catch your girlfriend in a lie, start gathering evidence about what you think she's lying to you about. The more evidence you have, the more likely she will have to admit to it.