Check the tire, it'll say on there. I figure go with the max reading.
If your trailer is a dual axle then make sure you stay with the same size tire for all four, including the spare of course. If one tire is smaller then it puts more work on the larger tire and causes the larger tire to fail. For the same reason you should check the psi at every fuel stop and keep the pressures identical. I fill an air tank before my trips.
Grease the bearings. If they go bad on the road you're really screwed.
These things have happened to me. Trust me, maintenance your trailer.