How can I tell if a breast lump is cancerous?

You can't. You need a physician to see and examine you and order the proper tests!
Biopsy. It is often very difficult to distinguish between benign breast lumps and cancer by examination alone. While mammograms and ultrasounds may provide some useful information, the only way to be 100% certain is by performing a biopsy.
Mammogram & sonogram. Mammogram and ultrasound are first steps. Majority of lumps are cysts , non cancerous glandular tissue or fibrocystic change. Mammogram may or may not reveal lump, but ultrasound can distinguish a cyst (fluid pocket) from a solid mass. The majority of solid masses, especially in a woman less than 40 years old, are not cancerous.

Related Questions

How can you tell if a breast lump is cancerous?

Breast Biopsy. It is very difficult to distinguish a benign breast lump from a cancer for patient and doctor. Depending upon age and physical examination findings, we will often get a mammogram and ultrasound to help evaluate a breast lump, sometimes culminating in a (nonsurgical) needle-biopsy--ultimately, the only way to be 100% certain is to look at the tissue under the microscope. Read more...

How can I tell if my breast lump is a breast cancer?

You need a biopsy. An experienced gynecologist or breast surgeon can often tell if there is a high chance that it is malignant or if it is likely benign. Regardless, any lump needs to be evaluated with a mammogram and possibly an ultrasound. It should also be biopsied. Read more...
Get it checked. The only way to know is to get it checked. Mammography or ultrasound may show the answer or you may need a biopsy for unknown or questionable lumps. Most can be diagnosed with a core needle biopsy which does not require going to surgery. Talk to your doctor to see what is best for you. Read more...

What are the signs that a breast lump is cancerous?

Variable. You can't tell by yourself and many times even breast experts can't by exam alone. All breast lumps that persist for more than a couple of weeks need a physical exam and typically a mammogram and/or ultrasound. Read more...
Hard to Tell. It is very difficult to distinguish a benign breast lump from a cancer by exam alone, for patient and doctor. In general, cancers tend to be rock-hard, 3-dimensionally round, and can feel "fixed" within the breast. Depending upon age and physical examination findings, we will often get a mammogram and ultrasound to help evaluate a breast lump, sometimes culminating in a (nonsurgical) biopsy. Read more...