As Chuck says, the way that virtually all of us use to check AquaHot level of Century Boiler Fluid is a refractometer. Available for under $30 from a bunch of sources, you can find refractometers which measure only one concentration or refractometers which have three or four different scales which can measure three or four different concentrations. I have both kinds in the tool box on my Cornerstone.
I often buy my Century Boiler Fluid in the concentrated form which needs to be diluted with distilled water. You can also buy it premixed.
Gary and Dee, Zowie and Bowie (traveling cat sibs)
2019 Cornerstone 45B, X15-605hp, Imperial, Spartan K3,
2013 Honda CR-V toad, Demco Excali-Bar II,
Demco Baseplate, Demco Toad Light system, 73 de W5FI