If there is one, the front plate would be behind the oversized load sign. Not all states require one on the front, though. The rear plate would be on the truck. The trailer might or might have a plate depending upon the state. If it has one, it would be placed somewhere so that it doesn't get damaged when the bed gets extended and tilted. I could be that its a self-adhesive label attached on the left side of the trailer or its tongue.