Biopsy is really the only real way they can tell if tissue is diseased, as they look at it under a microscope to look at the cells. Cancerous cells look different than normal cells. They can also tell which grade the cancer is by how different the cells are to normal cells.  The more they resemble normal cells, the lower the grade of cancer. I think that's how i remember the convo with the oncologist going.

Things can look suggestive of malignancy on scans and to the naked eye, but they can still be wrong. Works the other way too. Things can look harmless, but can change once looked at under a ms.