According to the American Cancer Society, women should be aware of their breasts and what is normal and abnormal by conducting self breast-exams each month.
Below are resources both telling and showing women how to properly perform a self-exam:
American Cancer Society: How to examine your breasts
Susan G. Komen: Breast Self-Exam
National Breast Cancer Foundation, Inc.: How should a breast self-exam be performed?
BreastCancer.org: The Five Steps of a Breast Self-Exam