What is the way to check if the air system of a trailer is fully pressurized?

The correct way to check if the air system of a trailer is fully pressurized is by opening the emergency line shut-off. By opening the emergency line shut-off valve, you allow the air to flow through the emergency line to the trailer. If the air system is fully pressurized, you should hear a rush of air or feel air pressure at the rear of the trailer when the valve is opened. This indicates that the air has reached the trailer and the system is fully pressurized.