Just like any other kind of interpretation, reading a mammogram is a skill that radiologists develop over time. They look for any signs of abnormality, including:

• asymmetries (something on one side that's not on the other)
• irregular areas of increased density
• clusters of small calcifications
• any area of skin thickening

But most of the time, a radiologist can't say for sure whether "it's cancer" or "it's not cancer" based on a mammogram alone, because both cancerous and non-cancerous growths can...