Which method is NOT typically used in testing trailer brakes?

Checking brake fluid levels is not typically used in testing trailer brakes because most trailers, especially those designed for commercial use, operate using air brake systems rather than hydraulic brake systems. In air brake systems, you don't have brake fluid to check; instead, the system relies on compressed air to engage the brakes.

On the other hand, releasing the air supply control is part of a test to ensure that the trailer brakes can be engaged and that the air system is functioning correctly. Using the hand control (trolley valve) involves testing if the trailer's brakes will respond when the driver uses the manual control, which is crucial for checking the responsiveness of the trailer brakes. Additionally, applying the foot brake checks the entire braking system's functionality, including both the vehicle and trailer brakes, ensuring they are working effectively in unison.