Share your intentions with the girl you're dating. Do it relatively early on, not six months into seeing her. This means explaining your expectations and hopes not only for your own future, but for your future together. Do you hope the relationship grows into something long-term, or do you plan to split up when you move across the country for school soon?
Talk instead of making assumptions. Different activities and gestures mean different things to different people. Meeting each other's parents or getting physically intimate automatically equates to a monogamous relationship to some people, but it might mean casual dating to others. Remember, sharing intentions doesn't mean you have to know specifics like whether this girl is right for you now or is the one you want to marry down the road. It's about disclosing the direction you're going, not necessarily the destination. Where are you headed? Answering this question helps avoid miscommunications, hurt feelings, lost time and bruised egos.