A thorough exam of your neck and shoulder areas will rule out these possibilities. If the numbness doesn't happen, than a reverse Phalen's test is done. Again, the numbness should occur in less than one minute. Tapping the wrist over the median nerve may also produce tingling in the first three fingers. Thumb strength is also tested.
Any weakness indicates nerve impairment.
In this test, needles are inserted from the shoulder down to the wrist. A small electric current is then sent through the arm. CTS is present. Further, this test shows to what degree that CTS is present. A MRI or CAT scan is rarely used, unless another condition is suspected.