Amount (percentage of sodium in meat being cooked) of sodium left in water (broth) after cooking meat
You have 2 options to measure the amount of salt in the soup. Measure the salt content directly by weight, or measure the electrical conductivity of the broth.

For either solution you need to filter the soup as much as possible preferable until it is almost clear.

Solution 1: Cook the filtered soup until it is dry and stops smoking, weigh the leftover residue that would be mostly salt.

Solution 2: Get a meter that detects the mineral content by testing electrical conductivity. Get something like this http://amzn.to/2u8pdwM and you can calculate the approximate amount of salt from that reading.