A hydrostatic test is required on a SCUBA cylinder every 5 years with a Visual Test required at the 2.5year mark. As part of the test, your valve will be serviced and the threads on the neck of the cylinder and the valve will be checked with calibrated thread gauges.
 
The test involves pressurising the cylinder to its test pressure (usually 5/3 or 3/2 of the working pressure) and measuring its volume before and after the test. A permanent increase in volume above the tolerated level means the cylinder fails the test and must be permanently removed from service.