I can't tell if my girlfriend's lying for my benefit?

Sometimes, girls show that they are satisfied in different ways and this may just be one of them. The other thing is that you may have to look at her and go "Hey, I know I'm not the best I can be at this so give me a few pointers". Another thing you can do is look it up, what are the pleasure centers and what will give you the best results. And although its awkward, ask your guy friends, they might be able to teach you a few tricks. good luck to you.