With engine cool and a voltmeter set to 1V D.C. resolution, stick the positive probe into the coolant neck and immersed a good portion of the probe into the coolant. With the negative probe, touch the battery ground or a good ground on the engine, read your voltage.
With 30K miles on the Honda blue coolant, I got 0.269 volt.
With 7 qts. new Honda blue coolant, I got 0.230 volt.
This surprised the hell out of me because on my 4 cylinder Toyota, it was initially 0.28 volt then after coolant replacement, I got 0.169 volt.
On my RX7 in which I run a coolant that does not have any water at all (Evans NPG+) I got 0.069 volt after 5 years and 30K miles.
The lower the voltage the better because there is no cathodic reaction between dissimilar metals with the coolant acting as the electrolyte.
What are yours with miles on the coolant etc..?