Have you noticed any kind of distancing behavior? This could be physical distance: Your partner seems to be making less time for you; you seem to do fewer "couple" activities you used to enjoy. Emotional distancing is more subtle. Lack of eye contact, changes in the way your partner communicates, a sudden reluctance to make plans - these are all signs your relationship is on the rocks.
A New Routine
Be wary of sudden changes in your partner's daily routine. Maybe he finds himself too busy to do the things you used to enjoy together. Or she's recently surrounded herself with a crowd of single party girl pals. A new change in hobbies - particularly an obsession with working out or dressing differently - can sometimes mean he or she plans to re-enter the dating scene soon.
You Fight More
Some cowards would rather pick a stupid fight hoping you'll break up with them than do the dumping themselves. Are you having overblown arguments over the proper way to make the bed or why you put the empty milk carton back in the fridge? In more serious arguments, they'll be more likely to see where you're wrong than where you might be right.
But You Might Also Fight Less
Don't rejoice if your regular spats seem to disappear completely. A sudden reduction in the frequency of your fighting can mean that your partner has started to pull back and minimize his or her emotional investment. In short, they just don't care anymore.
The Sex Is Different
Sure, you'll be suspicious if your sex life falls from frequent to forgotten, but be just as wary if your lover shows up with new tricks and a trapeze with the tags still on it. A dramatic change in sexual behavior may mean he or she is trying to avoid situations that include the expression of emotion or attachment...or that your sweetie's getting it on the side.