If the tires are original, you can get an approximate value. The tires(trailer tires) have a weight rating on them. Normally, the rating of the two tires added together will be a little higher than the gross weight capacity which includes weight of the trailer itself and loaded boat. If you know the manufacturer name, they will be able to give you a definitive answer. If they are not the original tires, I would not trust any number you derive from them. Good Luck and remember to open up the bearings to assure they are ok if it's a used trailer.