The best way to determine the value of a used horse trailer without living quarters is to contact a reputable trailer dealership and have that company determine the value. If the trailer has living quarters, you can usually find it in a value guide such as NADAguides.
There is no standard guide for valuing horse trailers like there is for other types of vehicles, although as of 2015, some organizations are trying to put one together. You can also get an approximate value by doing market research, including looking at advertisements for similar trailers for sale. This method generally works if you are selling the trailer or buying it with cash and trying to make sure the price is reasonable, but it is not reliable enough to secure financing in most cases.
Trailers with built-in living quarters are generally considered recreational vehicles by organizations such as the National Automobile Dealers Association and are included under the RV or travel trailer section of their guides. There are a handful of other trailer blue book guides, including one at horsetrailerbluebook.com, that can help you get an idea of the value. The North American Trailer Dealers Association may be working to put one together, but as of 2015, it is not available to the public.