I've been in the coolant business most of my life (1 formulation patent that was in use for 15 years, and have produced many millions of gallons).
Testing for glycol is simple. every sailor should have a pocket refractometer, since it is good for battery
SOC too and is very accurate. Also good for making certain that winterizing glycol is sufficient (you never know how much water
was in the system--test what comes out).
pH will tell you nothing unless you know the formulation. Additionally, the range of acceptable is quite wide.
The MOST important spec is chloride, from seawater contamination. No field test accurate enough in the low range. This is the reason Yanmar
recomends annual changes--the seawater risk.
There are test kits available for $15-25 that give meaningful information, but unless you know the chemistry well and use them regularly, they are only confusing.
Short answer; change the AF annually and don't bother testing. The only reason I might consider testing is if the AF looks really strange and you want to know what is wrong... but will you know the cause from the data? Not unless you are in the industry. A lot of rust generally means air is getting in the coolant, generally from an exhaust gasket
pump. Pitting in the heat exchanger
is generally a salt