A healthy snake should be alert and active and have a well rounded body with no scars of any type or scabby looking scales, and they should not have old shed stuck on them. You should always ask what they are currently eating ( live or Frozen Thawed) and if buying a hatchling you should know how many meals they have had , and when their last shed was. You should not buy a snake that has not had at least 3 meals unless you are experienced with newborn snakes. I reccomend for your first snake, getting one a bit older and already established...maybe a yearling.
At the shows some vendors will take credit card its an individual thing. They should allow you to hold the animal and answer all of your questions about that animals. They are not there to answer all your care and husbandry questions though. You should study and get those answers on your own by a book such as Kathy Loves Manual, and by researching on the internet. Remember the seller should give you ample time and attention in regards to seeing the animals and finding out more about them, but they also need to give that attention to others .
There are many other points to picking out a good animal but these are the top ones I have thought of for now.