What is the best way to test the adequacy of a generalization

The answer is: Face it with more examples of the same kind.

Explanation:

Adequacy means to be "satisfactory" and generalization means to be simplistic. So in this case, a good way to actually measure how satisfactory is a very general comment, for instance, is to actually think and give more examples of that same comment/idea, so that it complements and actually evaluate if the idea is congruent and good enough.