Found this information in another Forum so thought I would borrow the info and pass along this handy info.
"The Samsung Galaxy S3 has more to it than visible to the naked eye. If you like to keep a check on your phone’s specifications, then this is something that is going to work as your best diagnostic tool. Interestingly, Samsung Galaxy S3 has a hidden service menu that will aid in testing the specifications of the device,
Here is how you access the hidden service menu on your Galaxy S3:
1. Go to the dial-up option from your Galaxy S3 home screen.
2. Now, type in: *#0*#
Once you have typed in the above mentioned digits, you will be able to see a grid of options (5x3). The tabs available on the service menu will enable you to test the following:
1. Color capabilities of the device
2. Receiver and Vibration
4. Mega cam
5. Various sensors including Proximity sensor, Lights sensor, Magnetic Sensor, Barometer sensor, Gyroscope sensor
6. Touch screen of your Galaxy S3
9. Sub key
10. Front cam
12. Low frequency
Samsung never declared the built-in testing ability on this super phone. Obviously, this is for the use of technicians to quickly check out which specification of the phone is not functioning properly. However, it is useful for any Samsung Galaxy S3 owner to have know-how about this service menu so as to check the device for optimal functioning. Go ahead and check the hidden service menu on your Galaxy S3."- GalaxyPro