There are many convincing reasons not to rely on your breasts to tell you when you are fertile, however. Breast tenderness around the time of ovulation is caused by the surge in hormones going on in those days. Your endometrium, the lining of your uterus, is readying to welcome a fertilized egg as well, for instance. Breast tenderness may occur before, during, or after ovulation. The feeling of sore or tender breasts is highly subjective and can even be the result of some medications. If you are relying on your breasts too much, and not using other methods like ovulation tests to tell you that you are fertile, you could miss your fertile window altogether. When determining when you are ovulating on the basis of physical symptoms, taking a whole-picture approach is certainly the most effective.
Look out for other signs like ovulation pain, egg white and slimy cervical mucus, and sometimes ovulation bleeding or spotting. When your body does not give you any clear clues, relying on other methods will give you much more information. Speaking from personal experience, I think ovulation tests and an ovulation calendar are the way to go. But many women do great with measuring their basal body temperature (BBT in fertility speak) as well. And of course, it's worth remembering that women who have no clue when they are ovulating can get pregnant as well!