How do you diagnose kidney cancer (renal cell cancer)?

X-rays. The vast majority of kidney cancers are diagnosed soley on radiological findings (x-rays). These tests include a ct scan and/or MRI and chest x-ray. In addition, other tests such as blood tests are also used to help aid in the evaluation of metastatic disease. Kidney biopsies are rarely indicated to make the diagnosis.
Removal. As mentioned by others, it is usually not needed to biopsy a kidney tumor once it is found by imaging, since renal cell carcinomas have a characteristic appearance by x-ray studies. However, a small percentage of these tumors once removed are found to be benign (noncancerous). So the final tumor type is not 100% confirmed until it is removed.