You want to make sure both parents have been shown and both won conformations (titles), that both parents have been temperament tested, heart holstered, hips, eyes, and other blood tests ran as well, typically a reputable breeder will not own sire and dam (not always but 99% of the time), and reputable breeders will want to know something about you, possibly want to speak with your vet, see pictures of your home, info on past dogs, your experience with dogs specifically Boxers they won't just allow you to come in and take a puppy home without getting to knwo you first, they should also offer a health guarantee of 2-3 years and be willing to take the dog back for any reason for it's entire life. Also if the puppy is being sold to you as a pet only there should be a spay/neuter claus somewhere in the contract.
If they do not have both parents shown and have won titles (in the show ring), they do not have all of their health testing (x-rays aren't enough), temperament testing, a health guarantee, and be willing to take the dog back for it's entire life then stop right there and run away. Some of the other stuff can be less important but those are the top things and if one's missing they may be "getting there" but I still wouldn't purchase from them.