but no easy answers!
Anyone who has been breeding corns for a long time will know their own bloodlines pretty well, and have a pretty good (or very good) idea of what they consist of. BUT - even in some lines that I have bred since the late '80s or early '90s, I still outcross now and then for new blood. And even if I see the parents of new acquisitions, it is not likely I will see the grandparents and great grandparents.
There have been a few times that I was very interested in a new baby or two at a show because it was very unusual for one reason or another. But upon closer questioning (or inspection), it became apparent to me that the unique baby was the product of a little bit of kingsnake or other "exotic" influence. It can be very subtle if it was a couple of generations ago. If it had been even less of a percentage, nobody could have been very sure of it. And that is how hybrid blood can be introduced, although after more generations of breeding into corns, the percentage will be very, very small. But it is still there, and as others have said, it could be anywhere, even possibly in a wild snake, or more likely, snakes at shows and shops.
Also, it can be difficult to say for sure what genes are NOT in a particular line, if the breeder has done some outcrossing. For example, I have used hypo so much in my bloodred projects that I usually just say that any of my normal bloods COULD be het for hypo. OTOH, my use of other genes (charcoal, amel, anery, etc) has been fairly restricted in my bloods, so I can FAIRLY well declare that my normal bloods are free from those genes unless they came from certain clutches. That is just one example. Any longtime breeder will be able to tell you which bloodlines he / she has worked with for a long or short time, and what the likelihood is of unexpected results. But something from many generations ago that was thought to have been "bred out" by now, can still pop up unexpectedly from time to time.
Because I have worked on certain lines for a long time, I also know that way back in the history of my candycanes, a couple of light colored creamsicles were used as founder stock in the late '80s or early '90s. That means that abut 15 - 20 years ago there was an injection of part emoryi blood in the new morph that was to become candycanes. Of course, back then, emoryi was classified as a subspecies of corn. Now it has been given its own species. So now my modern candycanes probably still retain a couple of percentage points of emoryi in them. It is not a secret, for those who care. But of course, some that come from shows may or may not have more or less emoryi - no way to tell. So you have to decide if it is important to you. If some possible emoryi blood (even a few percentage points) is important, then don't buy my candycanes, or any from anyone who has not kept their own lines since the beginning of the morph. If you can't trace it back to the beginning, it may well have a little emoryi in it. And of course, that may be true of almost any other morphs as well.
But then I also know that my other long term lines that are not creamsicle crosses, do not have any creamsicle influence - unless it came sneaking in with some well screened new blood I acquired elsewhere - no way to know for positive.
I feel that the ACR (American Cornsnake Registry http://herpregistry.com/acr/index.php ) will EVENTUALLY help the situation. Although the info is only as good as the person who registers animals, eventually there will be a photo record for several generations online, for each registered snake. Then a potential customer will be able to look through the photos and see what morphs (or if there was a non corn in the genetic background) and decide for himself whether the bloodline meets his needs. The registry is still pretty new, although they have over 5,000 corns registered. It will take a while before many animals have 3 or more generations with photo records. But it has to start somewhere - the sooner the better. Take a look at it if you are not already familiar with the project.
As others have said, if you want a pet, just get the best deal on a healthy animal that you like. If you want to breed and know genetic lines, buy from a long time breeder who can answer some questions about their bloodlines. Even longtime breeders will have better answers for some lines than for other lines. So question carefully and choose according to the answers you get. And consider supporting the ACR for the future of the hobby.