You can get test strips for coolant that show the acid level. GM's Dexcool coolant is a particular pain because it likes to do weird things like turn to jello and clog coolant passages when it's old, which is why you're well advised to not let it go much over the 5 year recommendation, in addition to acidity problems of older coolant. It's also doesn't like mixing with conventional coolant so you need to ensure you use Dexcool or a compatible coolant when adding or replacing unless you do a thorough job of the flush.