Some cancers you can see, for example skin cancer. Others you can feel, like breast cancer. Sometimes the cancer is deep inside the body, but there are things like urine that can be tested to see for signs of cancer.
If doctors think that a person has cancer, they can do an Xray or a very powerful Xray called a CT scan. If they see a mass of cells that might be a tumour they can perform surgery to take a bit of the cell mass out. This is called a biopsy. They can then stain the cells in the potential tumour to see if it is cancer.
i personally test for cancer from getting biopsies or excisions from patients (the cancer is cut out by a surgeon). i further cut them up and look at them under the microscope.
Patholgists can see the differences in the cells- they wont look normal- sometimes they have a bigger nucleus or they will mimic others cells form another part of the body. we can then stain them differnet stains to subtype for cancer. this was we can work out the best treatment for the patient