A good refractometer (sp?) is useful at checking coolant or battery fluid condition.
I personally use an old MAC hydrometer to check coolant. I usually only use it to check the mixture % after a flush, and not really to determine the health or condition of the coolant (use age or mileage).
IIRC, coolant types (ethylene vs propylene glycol) will test differently, so make sure you have the correct tester.