If somebody had kidney cancer can doctors tell what stage it's in by doing an mri or cat scan on the patient?

Not thoroughly: For example, ct or MRI of the abdomen would not fully stage because imaging of other parts of the body are needed to rule out metastatic disease. Nccn guidelines recommend the following for kidney mass work up: history & physical exam, cbc, cmp, ua, ct a/p or MRI and, chest imaging, & consider brain mri, bone scan, urine cytology, needle biopsy.

Imaging good info: You want to know if imaging studies like ct scan or MRI will help determine the stage in renal cancer. You will get good infrmation from imaging and the staging will be estimated. If the renal cancer extends into the inferior venae cavae or to the right atrium it can be detected by imaging studies. Surgery will confirm the clinical staging. Be well and i wish you the best.