Though healthy, frequent sex is a key part of a romantic relationship, it's not the only part. If the relationship were a house, sex would be a foundation -- it wouldn't be the kitchen, bathroom, bedrooms and light fixtures as well. One sign of lust is thinking of your significant other only sexually; you're not really interested in communication, intimacy or other parts of a relationship, but just want to have sex all the time.
People who are in lust tend to spend all their time together having sex. If you're going out with someone on the weekend, having dinner, seeing some friends, then coming home and having sex, there's probably more to the relationship than lust. However, if you both do your own thing, seeing your own friends then meeting up in the evening, what you are feeling is lust. People in love relationships spend time together doing non-sexual things; if you don't do this, then you are probably in lust.
Lack of Knowledge
Lust is physical attraction. You don't really need to know a lot about someone to lust after him. Rather, you just need to like the way he looks and like the physical chemistry you share. Because lust is just physical attraction, you don't really need to know any other details to feel it. If you are seeing someone and feel strongly attracted to him, yet don't know what he does for a living, where he's from or even (in extreme cases) his last name, you are well and truly in lust.
Everyone has an idea of what her future ideally will look like. If you are seeing someone and thoroughly enjoy your sex life, but, when you look forward, do not see her playing a role in it, you are probably in lust. Love is about keeping someone around for the future; lust is about enjoying yourself right now.