How do i tell my actual weight on my rear axle?

Loads placed on the trucks axles is measured at the tires and ground interface and includes axles/wheels/tires/etc.

Just for example.... Your 3500 SRW Dodge has a 6500 RAWR and many unladin rear axles weigh in the 2900-3000 lb range which leaves around 3500-3600 lbs for a payload.
When you have your trucks axle weighed separately keep a copy of the scale ticket in the truck for future reference.