As B&O suggested, you need to keep on top of your trailer tyres, especially if you're towing good distances. Obviously it'll affect the towing characterisctics, but under inflated tyres will also have a greatly detrimental effect on your fuel consumption
TBH, your tyre rating will all be down to your tyre type / size / rating. Here's a previos post on identifying the spec of your tyres: Trailer & Towing info - mods to make sticky!
As suggested, SBS should be able to guide you. However, they sometimes put the psi rating on the chassis plate too, so that may be worth a check. You should find it n/s front on the drawbar / a-frame. As others have said, the rating should also be on the tyre too. It'll say something like "60psi cold", or similar. Check there too.
If you don't get any joy let me know the tyre info and I'm sure I'll be able to assist.