There are a couple of numbers you have to be concerned with.

Being you have a 2500 - GCWR (Gross Combined Weight Rating - the total weight of truck + trailer) is not one of them.

Number one, as mentioned, look on your door jamb and see what the max rear axle weight rating is.

Next, determine your max payload capacity. Take your GVWR (Gross Vehicle Weight Rating) and subtract your actual truck weight (take you truck to a certified scale - truck stop - loaded as you would for a trip, include a full tank of fuel and passengers at a minimum IMO). Subtract actual weight from GVWR and that will be your actual allowable payload capacity.

The scale should give you a printout that shows your total weight and the weight on each axle. You should not exceed the GVWR or the maximum weight on the rear axle.

Most 5th wheel manufacturers will list the pin weight on their website, though this is usually a little low IMO as they are weighed in a base configuration before options are added.

With a single rear axle on a 2500, you are much more likely to exceed either your max rear axle limit or your load capacity before you reach max combined weight

 

If looking at trailers older than 2004 I think, the dry and pin weights were all but a guestimate since the dealers weren't required to weigh the unit as it sat on the lot. The manufacturers often listed weights way under the actual # to sell units. On anything newer, the dealers are required to modify the tag for any equipment they add-on themselves. Takes a lot of the guess work out of it but yes a run to the scale is always 100%. Same goes for the payload tag on any truck newer than 2006 I think. It'll list 'the weight of the occupants + cargo can't exceed XXXlbs" but again that won't include what you add on but at least everything that's from the factory on that truck with a 150 driver or whatever.