How does a doc find out if a woman's breast tissue is too dense for an accurate result on a mammogram? can he/she know before the mammogram?

On the mammogram: By doing the mammogram, the radiologist is able to determine the breast density. Although dense breasts may make the scan a little more difficult, digital mammography can help in identifying lesions.

Seen on mammogram: Breast density can only be assessed on a mammogram. It is not always related to breast size or age.

Breast density: is a mammographic feature, it does not actually mean that the breasts are physically denser or weigh more, so it can only be assessed on a mammogram. Although the sensitivity of mammography for detecting cancer is decreased in dense breasts, mammography remains the standard of care for screening dense breasts.