Looking at the tumor under the microscope and seeing how similar it is to normal breast cells. The more a cancer's cells look like normal cells, the more likely it is to be a non-aggressive cancer. Poorly differentiated cancers look least like normal cells. They are more likely to be aggressive. Moderate differentiation is in between.
Most breast cancers have moderate or poor differentiation.
The type of cancer. This is most helpful in uncommon cancer types. Some, like inflammatory, nearly always are aggressive. Others like mucinous, tubular, cribriform, and papillary usually are non-aggressive cancers. The nucleus is the center of the cell.