The only way to know 100% for sure if you have breast cancer is by going to the doctor to get a mammogram done.
But you can feel your breasts for lumps and any abnormalities in the breast which can be a sign of breast cancer.
If you do feel around your breasts and notice some lumps or abnormalities it doesn't always mean you have breast cancer but you should make an appointment to see a doctor about it to be sure.