There used to be a tag on the side, but to back that up the number was stamped into the LH side of the frame in front. Often right where the snap up bracket was located, so removal was necessary to get to it. It could also be covered by many layers of paint? That's where I would go for it.
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake