Several methods have been used in the past to determine shot peening coverage. An inquiry prompted me to start a thread on the topic.
I have a short article in the library you can click here: http://www.shotpeener.com/library/spc/2008040.htm
I also found this on the web: http://www.saranindustries.com/shot-peening/importance-of-shot-coverage.asp
Dr. Kirk has offered a peening coverage predictor program: http://www.shotpeener.com/learning/coverage_predictor.php
You can also go to shotpeener.com Library and insert the text "coverage" in the search box.