With the hand valve on, you should test the trailer brakes by opening the service line valve at the rear of the trailer. When you do this, you should hear:

Air escape from the open valve.

You should hear air venting to the atmosphere when the hand valve is on and the service line valve is opened at the rear of the trailer. This lets you know that the trailer brakes are getting air when the service line is then closed.

Source: Arizona CDL Manual