How can doctors find out if a tumor is malignant?

Biopsy. A biopsy is the only sure way to tell if a tumor is malignant. A pathologist will examine the cells under the microscope and decide.
Biopsy. Sometimes the x-ray appearance or clinical picture will suggest cancer but as a rule you need a biopsy. A pathologist will look at the specimen and maybe do additional tests in order to confirm it is cancer and find out where it started from. In addition, molecular and gene studies are often done to the tissue to help guide therapy.