Wheel pressure can be indented into the steel wheel or a sticker on the front or back or even in the valley. If its in the valley the tire will need to be removed from the wheel to find it.
Lots of off shore made trailer wheels have no pressure rating ID on them. Then a 1-800 to the trailer mfg or their wheel mfg/supplier will be required.
Your OEM ST225/75-15 D tires carry a 65 psi at 2540 lb load.
I bet the wheels are the higher 75 or 80 psi rated. Many folks use the 75 psi in the E tires with no run hot/wearing the tires center issues.