If you find a lump in your breast 4 times out of 5 the lump will turn out to be non cancerous but a doctor can perform an ultrasound to look at the lump but usually you'll need more tests to determine if the lump in your breast is cancerous.
A mammogram though can tell whether a breast lump is cancerous or not.
Cancerous breast lumps tend to be more irregular in shape and may also feel firm or solid, and might also be fixed to the tissue in the breast.