A mouth guard is usually given to alleviate symptoms associated with grinding such as headaches and muscle spasms etc. To evaluate if the teeth are okay, you will have to go for follow up visits with your dentist, to monitor tooth surfaces. This happens over a period of months. If you feel some relief with regards to symptoms , then usually this is a good sign. Keep in mind, that mouth guards will need to be replaced every year or two, depending on the severity of your grinding, since you can grind through your guard.