A trailer tire has a load rating inscribed along the sidewall. The key to proper inflation is knowing how heavy the trailer is when fully loaded.
Near the driver's seat, there is a print out of the G.V.W.R., or Gross Vehicle Weight Rating. This is the maximum weight that the trailer should carry. However, to make sure the trailer's tires are specifcally inflated according to the current weight, you should take the trailer to a certified scale is recommended.
It is important to calculate the weight distributed over the wheels into the inflation values. If the weight within the trailer is a bit uneven, you should inflate the tires differently, but keep in mind that each axle should have the same tire pressure for safety concerns.
