You should find a label, usually in the kitchen area, on the back of a cabinet door or on the outside wall in a cabinet. You are looking for "UVW" That means "unit vehicle weight". That number is the weight of the trailer empty, as close as the manufacuter stated it back then. It is usually pretty close in newer trailers.
The best method if you can't find a label is to hook up and go too some scales;Alot of your bigger farmers have scales that can do the job,or you might can go by a set of scales on the interstate and see if they will weigh it.