If you are looking for an inspection for purchasing and looking at condition you would take to any mechanic or trailer dealer I am guessing they would charge you by the hour and check it as extensively as you wanted
In NY you have to have an annual safety inspection on a trailer, just like a car. You can take it to any mechanic who does inspections and it costs the same as your car inspection.
Almost no one does this though, since the inspection sticker is not required to be displayed anywhere on the trailer.
If you are looking for a true 'how is my trailer holding up, do the bearings need packing, are the brakes OK' type inspection I would ask a trusted mechanic if they know of a place that will do trailer inspections (they just might do it for you).