You need to ask
They can tell after they completed the test what the results are. You have to ask right then or wait to see the report. Your neurologist makes the definitive diagnosis based on his clinical findings and your subjective symptoms. It would seem though that they would have to have a series of EMG / NCS to determine if you were stable, deteriorating, or improving. I also discovered that the conduct of the test is somewhat of an art and can vary from one place to another based on their procedures and the interpretation of the doctor.
In my example, I have had three NCS / EMG that have all shown a numerical deterioration, but not enough to be statistically signifiicant.