Depending on what brand you're looking at, I've found that by Googling it I can search different dealers web site photos of that specific brand. Once in awhile they will show a sticker with the dry weight of the 5th wheel.
It doesn't happen very often but often enough.
I found personally that you really need to look at the stickered weight on the trailer itself, but only if you know what weight you typically load into your trailers.
I went from a TT to a 5th wheel. Consequently all the stuff that went into the truck went into the 5th wheels basement. I know what my tow/payload ratings are and didn't push them when searching for our 5er. I wanted to make sure I stayed under most ratings.
Knowing what I know now and seeing how my loaded gear affects the pin and axle weights I can take that information and apply it to the sticker.
I added roughly 14-1500lbs to the 5th wheel. I also added 415lbs to the pin weight. I can apply that info to my next purchase.
Northwood puts the pin, axle, CCC and GVWR on their trailers. Makes it easy to compute.
For someone starting out I think there are two ways to do it.
1st is to take the 5th GVWR X 20% for pin weight.
2nd is to take the stickered number and add 12-1500lbs, (which is about average for non full timers) and then X that by 20%