The door sticker on 2006 and newer TV is a great place to start, but that will be the MAXIMUM payload for that TV within the Manufactures GVWR.
The ONLY way to know what you can carry either within the GVWR OR the rear GAWR, is to load the TV up as it would be ready to go camping (as stated above) and go weigh it each axle separate, then do the math.
TV built before 2006, don't have the Payload sticker, only a VIN sticker with Max axle weights and the GVWR listed, and the minimum tire size required to get that rating.