The manufacturer of the rim is the only real authority for it's particular specifications. The DOT requires tire manufacturers to identify rims by size and pass that information on to their consumers, certifying individual rim sizes for individual tires.
Rim specifications for weight and air pressures are the responsibility of rim manufacturers and are certified by the SAE. The DOT does not require those values to be put on every rim. Some manufacturers do, most do not.
Automotive rims are seldom upgraded for more load capacity which requires more air pressure.
The DOT does require rim manufacturers to identify themselves as the rim builder by insuring that information is on each and every rim. Using that information and the rim part and model number will get you the rim's specs form the builder.
Here is a reference to get into the ballpark. Here the rim load capacity will sustain whetever pressure is require to get to its maximum load capacity.
http://recstuff.com/Trailer-Wheels.a...tm_campaign=7H
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