For livers and lungs a doctor schedules a procedure called a “biopsy” where they take a bit of tissue out of the tumors. They send that to a specialist called a “histopathologist” that looks at the cells under the microscope and can tell what kind they, are and look for things that tell them more about the cancer.
Tumor cells will tend to look most like the cells that they originally came from, so if you see liver-looking cells from a lung tumor, you know that’s where it came from (and vice-versa).
Usually it’s pretty easy, though. The original tumor is typically pretty big compared to where it spreads to, so it’s pretty obvious even before they look at the cells
Doctors might not take biopsies from risky areas where it could harm a person taking a sample.
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