Seriously. Yes the ratings in the manuals might be on the low side as a CYA measure on the part of the manufacturer, but what else do you have to go by?
NOTHING. That's what else you have to go by.
So who is willing to risk life and limb by pushing past the manufacturer ratings further and further until they experience a failure? The only way you're going to find that "real" limit is by going past it, in other words, making something bad happen.
Even the guys who knowingly and willingly overload their vehicles still draw the line at a manufacturer's rating. Usually, the tires. Why? Because as long as the tires stay round on the bottom you've got a shot at getting the truck off the road without killing anyone when something bad happens.
That said, one test to failure does not determine the "real" limit. The next one might fail at 1/2 the manufacturer's rating due to an unseen defect. The one after that might exceed the original test by orders of magnitude. Testing of dozens and an average value is necessary. Not exactly something the average guy can afford to take on.