Understand that people show their love in different ways. Just because your girlfriend doesn't show her love in a way that you think that she should does not mean that she has no feelings for you! You may think that hanging out all the time and being together every single minute possible. But perhaps your girlfriend shows her love to you by doing nice things for you, like walking your dog or giving you a ride to the airport at 4am.
Pay attention. If you have doubts about her sincerity when she says "I love you," look at the other cues. Does she say she loves you when reach a goal? Or does she only say she loves you when she wants something from you - whether that be a ride somewhere, a gift, an expensive dinner, or sex. Does she tell you she loves you at random times, just because she can't hold it inside? Or does she only say it when she's apologizing for being a jerk - again? The timing of her I love you's will tell you a lot about whether or not she means it.
Have a conversation. If she has never actually said she loves you, you can always ask her if she loves you or not or why she never says it back to you when you say it. It might be scary to initiate that conversation but you will probably be glad you did.
Remember: Actions really do speak louder than words. No matter what she says - or doesn't say - the proof is in her behavior. Everyone shows love differently. Some girls will want to hold hands all the time. Some may like to say nice things constantly to show their love. Some girls will want to spend a lot of time together while others would rather do nice things for you to show that they love you. At the end of the day, if a girl is kind and caring, thinks of you and does "little things" to show her feelings, you can sleep at night knowing that her love is real!