Load your rounds Min to Max load in .1 or .2 increments and shoot them consecutively at a target (the longer the range the more clear the results will be).
300yds/m would be a good starting point.
As your powder loads go up your bullet holes should climb, and at some point there will be 3 or 4 shots that group together and then the holes will start climbing upwards again (you may find 2 groups up the ladder)
Here's a link that explains it in detail:
http://kingfisher.0catch.com/guns/laddertest.explanation.html