Go with someone who has been around a while and has a good reputation. Still no guarentee but it stacks the odds in your favor.
Whatever you do, make sure the snake matches the photo ID. That is a bit tougher at a show since it's crowded. I bought a het albino a few years back and got paperwork. When I got home, I realized the snake in the photo was not the snake I paid for. I called and was told he would ship the correct paperwork. Never did and dropped off the face of the earth. Later I came to find out he only had a few hets but made a bunch of copies of the paperwork (easy to do with a scanner and printer
) and matched up CH babies with the copied paperwork. I have had a very expensive pet normal ever since.