Most of what you would ask a Boxer breeder is the same as what you would ask any other breeder. Some other things to make sure of is health testing on sire and dam (can iI get a copy when, can I see a copy before I put a deposit down), temperament, energy level, pedigree (can i get a copy, can i see it before I put a deposit down). Ask to see/meet the sire and dam if possible. Ask to meet at the location the pu is being wheped/cared for at. Is the pup being home raised? Champions in the line? Most recent is how far back? Oldest is how far back?
Hope this helps and good luck on your search...Your gonna have a blast!!!
I think in the breeding section I have a few posts that I made permanent that really are helpful. A lot of what I wrote are things that I got from a few different people, mostly breeders that I trust and respect....Thank you Hanna for adding the bit about longevity of the line, i believe I had left that out.