All you can do is to assure that the part has full coverage in the areas designated to be peened. Determining anything other than full coverage is impossible! Stating anything other than full coverage on the engineering drawing or your certification is of great concern! (both are done now on a daily basis throughout the industry) how would you prove there is exactly 100 or 200% coverage if asked?
Technically, if there is too much coverage, other problems such as rolled edges or silver streak will appear which would be cause for rejection.