"USUALLY" I have found the stamped wt code on inner back side of the rim. If it is not there, nor does it have a code of some sort, I would not go over initial wt ratings per say.
But as some would say, the rim is a tool that holds the tire, so unless it is really weak per say, it should hold a few more lbs of pressure, the question becomes, how much more? then do you really need 80 lbs in LR E tires in a typical DOT LD Class 2 truck? my sons have LR E tires in a 7100 gvwr GM, and a Toyota tacoma, but neither run more than 40-50 lbs, otherwise they do not get a full contact width patch of tire on the ground, tires wear out sooner than later, stopping is not good, handling worst etc. I only run 50 lbs on my sw 2500 I am driving, unless loaded, as otherwise, with 3500-4000 lb the ra alone, 80lbs is too much.
Marty