Well you can always study her body language. Or listen to the sound of her voice when she says that. Or you can study her eye movements, like if she looks down or binks alot, there's a chance she's lying to you. Even if she is kinda lying to you, she's only doing it because she cares and she doesn't want to hurt you. So if that's the reason you should be very happy and grateful you have a girl like that. :)
Girls always say "I like him for his personality", but chances are she wouldn't have even considered to get to know his personality if he didn't look like a cute guy to chat up so don't worry. Girls who think your ugly won't kiss or touch ya.