(I've worked in the AF industry for many years, including one formula patent)
The simplest advise is to get a pocket refractometer, which reads EG/PG and battery
SG also. The SG (ball) units have serious reliability
issues and are no longer considered a good method. (freeze point correlates to both SG and index of refraction) However, you need to know which type it is (EG/PG), and color is not a sure indicator.
Freeze protection does not degrade with time, only with dilution. Corrosion
protection does degrade; typically many years, but certain engine
defects can ruin antifreeze within weeks.
RobinAir Coolant and Battery Refractometer ROB75240: Advance Auto Parts