coolant ok?

The service/maintenance schedule for my 98 Ranger says antifreeze/coolant should be changed every 3 yrs or 36,000 miles. The vehicle has 97,000 miles and I know the coolant has been changed periodically over the years, cooling system even backflushed at least a few times at times of coolant change. I have one of those cheap little coolant/antifreeze testers with the bulb on the top and the balls inside that will give you an indication of strength. I checked today and the got five balls. The coolant looks generally fine, nice and green, and the coolant level is good and all that. Trouble is, I didn't keep track of the mileage when I did the last coolant flush/change, and can't recall whether its been over three years either. So even though the coolant looks fine and apparently the strength is fine, is there a way I can test the actual condition of the coolant to see whether it's other protective qualities have actually deteriorated or maybe getting close to that point where it probably needs changing again? Any comments/advice appreciated.

Coolant breaks down over time due to heat and impurities in the engine block and peripherals; radiator, water pump, heater core, etc.
It may look okay and may not freeze up on you but it most probably has lost any lubricating qualities it once had.
Unfortunately I know of no way the common man, like you and I, can test it to see if it really needs replaced.
Anti-freeze is expensive, but if you're doing the work yourself, you are better off replacing it at the suggested intervals and flushing any impurities out of your system at the same time.
There are various inexpensive treatments available at Auto Parts places that you run in the engine for a short time allowing the chemical to dislodge what it can. Then, when you flush the engine you do a much better job.

PS. Sorry for the late reply to this question, I only joined the Forum over the weekend.