Towing Issues: Trailer Question, homemade trailers, tire condition

Question
We bought a homemade trailer that apparently used to be a boat trailer, now with wooden floor and sides.  Is there a way to tell what the carrying capacity is?  It has no labels any where stating the capacity.  It has a square axle and 4-lug tires, I believe the tires are 13s.  The total trailer length:  Without tongue - just over 10 feet, With tongue - about 13 feet.  The diameter of the axle is almost 2 inches.  I would really appreciate any help.  Thanks.

Answer
Its not really my area of expertise, but with homemade trailers so many things come into play...

But go look at the tires.  You should see some information on them which is the load capacity of which "EACH TIRE IS RATED AT" to carry a load safely...
If it says that the load capacity is 500 pounds then each tire is able to support 500 pounds before it goes over the rated capacity and has a chance of failure..

Now this also can be affected by tire condition, new, dry rotted, wore out and such...

That is just a simple way to do it.. Not sure what else to suggest.