Researchers use 2 main types of studies to try to determine if a substance or exposure causes cancer. (A substance that causes cancer or helps cancer grow is called a carcinogen.)
In studies done in the lab, animals are exposed to a substance (often in very large doses) to see if it causes tumors or other health problems. It's not always clear if the results from these types of studies will apply to humans, but lab studies are the best way to find out if a substance has the potential to cause cancer in humans before widespread exposure occurs. Another type of study looks at cancer rates in different groups of people. Such a study might compare the cancer rate in a group exposed to a substance versus the rate in a group not exposed to it, or compare it to what the expected cancer rate would be in the general population. But studies in people can sometimes be hard to interpret, because there may be other factors affecting the results that are hard to account for.
In most cases neither type of study provides definitive evidence on its own, so researchers usually look at both lab-based and human studies if they are available.