Ran across this on the bob is the oil guy forum. There’s a note half way down the posts stating:
So, test the corrosion protection. With a digital voltmeter, put one probe in the cool coolant in the radiator neck (not touching metal). Put the other probe on the battery negative post. If you read 0.1 volts DC to 0.3 VDC, you’re good. If you read 0.5 VDC or more, your coolant had depleted corrosion inhibitors and is due (or overdue) for a change. You’re reading the voltage generated by the galvanic action of the dissimilar metals and the electrolyte (the coolant).
That’s certainly something I never thought of. Hopefully it’ll help you out the next time you’re stuck trying to figure out if someone’s coolant is good and are short a hydrometer.