If i have a lump in my breast about the size of a pea, how can i tell if it is cancer?

See a breast surgeon: Self-examination definitely helps finding cancers but can be very difficult, since most women have "lumpy, bumpy" breasts; breast examination supplemented by ultrasound is the best way to distinguish the two. Other studies including mammography, or even MRI may sometimes be helpful. Ultimately, the only way to be certain is by tissue biopsy, which can almost-always be done under local anesthesia.