How to measure Sodium Hypo % on your own?

[newbie pete]

Anyone know how to get the exact sodium hypochlorite reading from chlorine on your own? We all spend a lot of money on this stuff and I'm wondering if anyone knows how to measure it on your own without taking the stores word for it. A store in RI was fined $10,000 for diluting their 12.5% before selling it to the customers and it gave me the idea of testing it myself

[jetstream1]

12.5% is fresh made so it all depends
if the store gets a full tank and it sits then it gets week
but if the store gets a full tank and sells as fast as they get then you are getting it fresh
I think the tag has to say 10.5 % when selling just in case it sits
Florida is a place were we get it fresh all the time