Wow! A state actually did that!

Here's how to test it yourself:

Add 1.88ml of your liquid chlorine to 5 gallons of water (1.88? Well, in Ben's PS-232, there's a 1.8ml mark so add just a tad more).

Or

Add 1 ml of your liquid chlorine to 10 liters of water (that's 5 2-liter soda bottles).

The test the chlorine level in the 5 gallons or 10 liters with a FAS-DPD chlorine test kit.

The measured level of FC is EXACTLY the concentration of your liquid chlorine. So if it measures 12.5ppm of FC, you 12.5%. If it measures 6ppm, you have 6% concentration.

Be sure to use ordinary tap water, not pool water. However if you DO use pool water, test the pool water first and subtract its FC from that of the final test water. So I tested my 5 gallons of poolwater and got 19.5ppm for FC. The poolwater directly was 5.5ppm. That means my 12.5% is actually 14%.