Consider who you turn to when you need moral support, advice or someone trustworthy to confide in. If you prefer to talk to a friend or family member before your significant other, this can indicate that she's not the most helpful, supportive or dependable person in your life.
Decide if you enjoy yourself more when you're with her or away from her. Although it's healthy to spend time apart, it's a bad sign if you'd consistently rather be around friends than your girlfriend.
Calculate the amount of time you think about other women. A little flirtation and fantasizing are normal, but battling the urge to date other girls (or worse, giving into temptation so you're no longer trustworthy) is a definite sign that your heart isn't completely into this relationship.
Imagine your future, including your personal goals, career aspirations and family life. If the girl you're dating isn't a clear part of the picture, you may not love her enough to envision a future with her in it.
Analyze your desire to "work on" the relationship. If you're indifferent to the girl you've been dating, don't care much about what she has to say or how happy she is with you, it may be the end of the line.
Search your soul to discover the reasons why you're still in this relationship. You may care for her as a friend, but not love her enough to make a commitment. Be sure you're not just with her out of habit, because you'd rather not be alone or for financial or social reasons.