It started on Sex and the City, and now it's about to be a major motion picture. Girls want to know just how you know he's just not into you (actually guys want to know the same things about women). In both cases, some clear rules apply. Here, with the advice of Dr. Bethany Marshal, are 10 tips on picking your way through this wilderness of a question. He doesn't plan in advance "If he's interested," warns Dr. Marshall, "there will be a future-mindedness to the relationship." This doesn't just apply to the long-term future. Obviously, if you haven't been dating for all that long, you're not going to talk about your plans for next year's Thanksgiving. But if he can't even book you a night next week, it's a problem. "Healthy relationships are always forward-moving," says Dr. Marshall. "Even if he's busy the next weekend, he will acknowledge his busyness and mention that he wants to see you at some point." He only answers when he doesn't know it's you It may seem obvious--clearly, he’s avoiding your calls. You should never have to trick a man into picking up the phone. It's easy to make excuses. (He has to answer no-caller-ID calls; they could be from work), but the bottom line is, he should answer your calls because he wants to, not because he has to. Another red flag, says Dr. Marshall, "He only calls from restricted numbers. And because you don't know his last name, address, mother or best friend, you cannot call information and get his phone number." He forgot your birthday ... Three years in a row So three years is a little extreme; hopefully, by this point, you've realized things are not going to change. But whether it's your birthday, your anniversary, your Friday night dinner date, or simply picking up the dry-cleaning that he's forgetting, when it happens more than a few times, it's a real problem. He doesn't have the consideration to think of you when you're not there. And Dr. Marshall points out how this can take a toll on you: "Birthdays and anniversaries become painful reminders that time is racing by and nothing has changed." Most time spent together is between midnight and sunrise Just like most living things, a relationship needs to see the light of day. Only seeing you late at night means you're not a priority. He's spending "normal" hours with other people, and knows that he can simply give you a ring when he's done with them. Once this happens two or three times, it becomes routine, and it's a difficult pattern to break. He doesn't mind if you "Date other People" Of course, when you first start seeing each other, it is totally normal to date other people. But after some time, if he wants to keep it casual and could care less what you do when he's not around, then it's time to say goodbye. "If he doesn't consider you a prize," says Dr. Marshall, "then he's not that into you." He dates other people Again, it is appropriate for both of you to "play the field" until a commitment is made. "But even so," says Dr. Marshall, "you shouldn't discuss your respective dating lives. If he's not connected to you, he will forget about you whenever you're apart." If he talks about other women to you, it could be a sign that he doesn't take your feelings seriously. He only wants to hang out on his turf Dr. Marshall says to really look out for this one. "This is a biggie!" If he's not willing to make the effort to come to you, then it implies that he wants you to do all the work in the relationship. "If he's into you, he will be interested in your home, your friends, and your life. If there is no interest, it is not a relationship." 90% of the time, you call him All relationships are built upon mutuality. It's important to take these little signs into consideration, and remember that you deserve better. Dr. Marshall advises, "If you do all the calling, he's not that invested. Step back, and put yourself on phone restriction. If he's interested, he'll call. If he's not, he won't." Yes, it will be painful when you realize that he hasn't called in a few weeks. But it will allow you to realize that you do, indeed, deserve better, and eventually, you'll be able to move on. He always has an excuse to get out of meeting your friends and family This relates back to the reality that if he's interested, he wants to be a part of what is important to you. He wants to meet the people who you grew up with, who raised you, and who you spend most of your days with. "If he's not willing to meet your family and friends," says Dr. Marshall, "it is because he does not envision himself being a part of your life." He told you, "I'm just not that into you" This may be the most important of all. It's easy to rationalize his feelings and tell yourself that he's going through a hard time and will change his mind soon, but, when it comes down to it, you're only making excuses. "Believe everything he tells you about himself--unless he is bragging," says Dr. Marshall. "In other words, if he says, 'You're not really my girlfriend,' believe him. Besides, if you think he’s a liar, then why are you with him?" Face it. You deserve someone better.