When starting the engine of a truck with a trailer, you can tell that your vehicle's ABS is not working if:

When turning on your vehicle's engine, the lights on the dash should come on and then turn off. If the ABS malfunction light stays on, the system is not working properly. In the case of a truck with a trailer, if the yellow light on the left rear of the trailer stays on once the engine has been started, the system is not working properly.