My advice - write down the questions you want to ask before going in, because as mentioned, in the heat of the moment you might go brain-dead and end up going home without knowing what your new snake was eating. Happened to me! Luckily the breeder I got Viago from was very responsive over instagram.
Also, ask to handle the snake your interested in! Most breeders will be completely fine with this and even welcome it, and it will give you an idea of how the bp does with handling. At an expo, with everyone passing by and being in tiny, unfamiliar containers, they're likely to be a little stressed. Keep that in mind when considering the snakes potential temperament. If a breeder doesn't allow you to handle the snake, that's an immediate no. Holding it will not only allow you to have an idea of their temperament but will also give you the opportunity to look the snake over for any injuries, scale rot, mites, etc. You don't need to do a full examination lol, but giving it a quick once over is a great idea.
Some questions I'd have written down:
What are you currently feeding it? Live or f/t? What size? How often?
How old is it/what does it weigh?
What genes does it carry/what produced it (parentage)?
Also read the breeder's body language/attitude. A good breeder will be happy to answer absolutely any questions you have and won't hesitate to provide you with contact information whether or not you've purchased from them that day. If they seem hands off or dismissive, I'd steer clear, that just indicates they may not be open to talking with you down the road should you have any questions, or god forbid, any issues with your new baby.