1.) Bedroom Blues
The best barometer to determine if a relationship is on the rocks is the amount of heat created between the sheets. Sex with someone you're into gets better over time. If things are stale and unexciting in the bedroom, it's a pretty good early warning sign. First thing: Consider the source for the lack of lust. Fights. Resentments, and guilt can all impact the quantity and quality of your nooky.
2.) Scheduling Conflicts
Are you having fun—together? You should be during the early weeks of the relationship. But if it starts to feel like a chore or you have to book so far in advance that you feel you're making an appointment with the dentist, that's a warning sign. If neither one of you is really making the time for the other, consider revising your relationship status.
3.) Molehills to Mountains
Little disagreements can be precursors of big problems to come. You should be arguing about which position to try next, not "why you didn't call?" If she is exhibiting signs of irrational jealousy, drama-queenitis, or just plain psychochick behavior, that's a warning sign. A molehill should be just that, but enough of them will form a mountain of aggravation.
4.) Supply and Demand
Are you getting what you need? In all areas, It's important to determine what you both want and what you are willing to give. You're definitely out of sync if she is daydreaming about building a Brangelina-like family and you call babies "rugrats"; or if you like to snowboard and she won't go anywhere without sand. These are red flags. Address these discrepancies, or you may wind up unsatisfied. In all areas.
5.) Panel of Experts
Of course, there will be the predictable jealous outcry from your buddies when suddenly you're not as available for a night out. And no girl will ever be good enough in your mother's eyes. But what are the people saying about how you are when you're with her? Are you happy? Or have you become stressed, apologetic, and, um, whipped? If your friends have nicknamed her "Sucubus" and your grandma refers to her as "that girl," that's a warning sign. Let the judges speak, and she just might get voted off the island.