If I didn’t apply an inert gas blanket to Part A right away, how can I tell if it has been rendered unusable?

There is not an easy way or direct indicator to tell if Part A has been rendered unusable.  The Part A in question should be mixed with Part B according to the application instructions and applied to a test piece and testing should be performed.  Testing procedures can be found at http://tinyurl.com/gentoofaq1.  If the test piece fails any of the tests, this may indicate that Part A has been rendered unusable.