As antifreeze ages and becomes more acidic, you can actually record a voltage reading between the copper radiator and coolant with a voltmeter. The higher the voltage reading, the more the necessity to flush and change the coolant.
Tudor is correct. You can also use chemically treated test strips that change color to indicate the pH of the antifreeze. BUT all a litmus paper reading really tells you is whether or not the antifreeze is alkaline or acidic. It doesn't tell you how much reserve alkalinity or corrosion inhibitor the antifreeze has left. Nor does an alkaline pH reading always mean the antifreeze is still good. When aluminum starts to corrode, such as with an aluminum flat head, the corroding aluminum can actually make the pH go back up!
This pH I'm referring to is a measure of the acidity and alkalinity on a scale, i.e., a pH scale. A reading of 7 is neutral. Lower numbers represent increasing acidity and higher numbers increasing alkalinity. Pure water is neutral with a pH of 7 whereas battery acid reads 2 or 3 on the pH scale while baking soda might be 10 or 11.
A low pH reading (below 8) would generally indicate bad antifreeze and a need for a change. But at the same time be aware that the newer long-life antifreeze can protect to lower pH, almost 7.
Now as Tudor suggested, you can also use an ordinary digital voltmeter. With the engine off, touch the voltmeter positive test lead to the radiator (must have good metal to metal contact). Open the radiator cap and insert the negative test lead down into the coolant. A reading of up to 0.2 volts is considered acceptable and indicates the presence of reserve alkalinity in the coolant. If the coolant reads 0.3 to 0.6 volts, it is borderline and should be recycled or replaced. A reading of 0.7 volts or more would tell you the coolant is overdue for a change.
Now having said all this, I'm trying to think if the following could somehow apply to a Model A, but corrosion in the cooling system can occur, regardless of the condition of the antifreeze, if voltage from things such as starters, ignition, etc., etc. flows through the coolant to ground rather than follow their intended ground path.
You can check for this condition using your voltmeter with the same method as stated above except do it while running the starter to crank the engine; then with the engine running and an accessory turned on such as the lights. If stray current is grounding through the coolant you'll get a voltage reading. More than 0.15 volts can corrode aluminum and 0.3 volts can be harmful to cast iron.
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