The time when a woman is preparing for the mikveh is a perfect time to do a breast self-exam. Mikveh use is generally at a consistent time of the menstrual cycle, she is undressed and inspecting her body anyway, and a mirror is usually available.
A breast self-exam is done in two steps. The first is to stand in front of a mirror with hands on the hips and observe the breasts for any change from the last examination as well as any striking difference between the sides (no one is perfectly symmetrical). The next step is to palpate (feel) the breasts in a consistent and thorough fashion, checking for any lumps. The most common recommendations are either in a circular motion progressing from the chest wall to the nipple, or going from chest wall to nipple as in the spokes of a wheel. If she finds a lump, or feels anything on one side that she does not feel on the other, she should consult her physician. Most such lumps are benign, but they should be checked to make sure. It is not unusual to have changes in the breasts during pregnancy and breastfeeding.
You can find more detailed instructions online. Some good sites are http://www.breastcancer.org/symptoms/testing/types/self_exam/bse_steps.jsp and www.komen.org/bse/.
In addition to breast self exams, a woman should have a breast exam by a medical professional annually. Women age 40 and above should consult their physicians about the recommended frequency of mammograms based on their family and personal histories.