Usually a Dr will start with a EMG and that's just tiny electrodes he places on certain areas on the part you are have your symptoms on. This test fires if you will a tiny amount of electrostatic thru these electrodes to see how that particular nerve reacts. Don't worry its not painful, just a funny feeing where the electrode is placed. Of coarse there are other tests, but this is the basic one. Please rate my answer
There are a battery of test, many which a neurologist will perform to determine nerve damage and the extent of it. Is this for yourself or family member? If so, check with doctor or clinic for a referral