I don't think there's really any way to measure tension so what guys default to doing is measuring the necks before and after and using that value for "tension". It's not really a measure of tension of course, but it's all we can reasonably measure and the thought process is that given consistent brass properties the tension will be the same between cases of equal sized neck diameter. This changes with firing as the brass is hardened so annealing is used to return the brass properties to as close to new as possible.
There are products like the 21st century hydro press that measure seating force which gives you a closer approximation of tension, but it's still not an actual measurement of tension. I think you could infer that rounds requiring the same seating force would have similar neck tension, but it's still not a direct measurement.
http://www.xxicsi.com/hydro-bullet-seater.html
