If you clearly identify a discrete mass, consider it to be malignant until proven otherwise. In general, determination of final diagnosis requires a biopsy.
A dominant breast mass that does not have a corresponding abnormality on Mammogram (i.e. "normal mammo") should still be considered malignant until proven otherwise. This is because not all malignancies generate mammographic findings.
While uncommon, breast cancer can occur in men. Thus, discrete masses should be appropriately evaluated.
Breast cancer can occur in young women (20s and 30s) �thus worrisome masses in this population should be appropriately evaluated.
If you have any concerns or uncertainty re any exam finding, seek input from someone with appropriate experience and training.
Pay very careful attention to any mass that the patient brings to your attention. Women who are good self-examiners can often detect subtle/early changes concerning for malignancy that an examiner may have difficulty identifying.