Abstract: The invention includes, in one embodiment, a method for determining the presence or absence of a corrosion-inhibitory level of carboxylate anion in used engine coolant which includes: obtaining a known quantity of the engine coolant as a representative sample thereof, the sample containing a level of carboxylate anion to be determined; adding a fixed quantity of an aluminum containing color indicator to the sample, where the color of the aluminum color indicator is different from the original color of the sample, thereby changing the color of the sample to substantially the color of the aluminum color indicator, where at least a portion of the aluminum color indicator complexes with at least a portion of the carboxylate anion to form a colored insoluble aluminum-carboxylate complex, where the colored insoluble aluminum-carboxylate complex precipitates, and where the color of the sample remains substantially the color of the aluminum color indicator when the level of carboxylate anion therein is below an effec