Couple of ways, if you have her papers, you can try a google search of the kennel names directly at the Parents and Grandparents, chances are the Great Grandparents may be too far back.
Is there a local Boxer club in your area? You could bring the pedigree to someone and they may know?
Good luck to you, we just ran into a neat story, I bred Dachshunds years ago, and the owner lost his 15 year old and on top of it, moved to British Columbia, well I found a breeder in the Seattle area and he sent me a copy of the pedigree, and low and behold, the Great Grandsire was our puppies Grandpa, they has used frozen sperm and his new pup is actually not directly, but is off the same line on the sire's side
Pedigrees are fun, maybe you will have such great luck as well...