There is most likely a sticker on the front/side of your trailer that states the minimum PSI. The tires sidewall will state the Maximum PSI. As long as you are anywhere between those, you are good. If you don't have the sticker on the side, you can use this chart to determine your minimum PSI, but you would need to know your axle weights.
Trailer Tire Load/Inflation Chart | Maxxis USA
Inflating to Max PSI is an easy answer that will get you by, but it is not necessarily the best answer.
Below is a document put out by Goodyear stating that you can increase your maximum speed by inflating up to 10 PSI over the minimum for a given load.
Personally, my trailers minimum is 50, my tires maximum is 65, I have been running 55.