Think about how you spend your time together. Do you have meaningful conversations or just sex? Flirty chit-chats don't count – be sure you're talking about things that matter to each of you, particularly your goals and future plans. Sex obviously enhances a relationship, but when the relationship is based solely on sex, then there's a problem. Be sure you're going on "real" dates and leaving the bedroom once in a while. Finally, a warning sign that it's just a fling is his avoidance of "the talk." Most guys aren't big fans of defining a relationship, but at a certain point, it needs to be done to prevent hurt feelings. If he avoids that conversation by changing the subject, then it may be time to chalk the relationship up to a fling.