There are three key elements to the valve: the actuator, the gasket and the valve body. Apart from leaking, valves may also suffer from other problems such as moving too slowly (plunger friction or stickiness), flutter & noisy valves, diaphragm leaks, gasket problems etc.
The actuator is quite probably going to be pneumatic operated, and both internal leaks (on the diaphragm) and external air leaks can easily be found. The gasket can be leaking, there can be mechanical problems on the plunger, such as friction or sticking, and there can be problems internally on the valve body itself. All these problems can generally be detected/checked by using ultrasound in the airborne or contact mode.