The safest way would be to take your tires (and trailer) to a reputable tire store in your area and have the store inspected the tires off the rims and get there professional opinion.
It would also depend on your plans for the next few months, winterize and store, then wait until next season. If you a planning longer trips, heat and speed, it might be the time to replace now.
If your going to sell the trailer, new tires would be a selling feature and you may get some of your investment back, if trading in ??.
Beverley and Ken