Ask how many times they've eaten, 3 in a row is typical minimum "industry standard" if you will, for ethical breeders to sell babies as established.
Ask what they are eating, as in live or frozen thawed prey. If you intend to feed one or the other, it's best to get one already eating what you want. (Many keepers recommend feeding frozen thawed, if the snake will take it.)
Ask when it hatched.
Ask what gender it is, even if you aren't planning to breed, it's a good idea to research corn snake reproduction a bit, as some behavior and health issues can be related to the snake reaching sexual maturity and can come up whether they are bred or not. So it's a good idea to know a bit about the process to be able to identify anything like that and while not strictly needed, it is more helpful in that area to know the gender of the snake. That's also often a benefit to buying one from a breeder instead of a pet store, as they typically don't know genders.
Look the snake over carefully, making sure it has no visible stuck shed, crusty or bubbly stuff around the mouth or vent. That the eyes are clear and not sunken. Look out for wrinkles in the skin. Those can all be signs of a health problem. A healthy baby should be shiny and have good muscle tone, it should move around and grip easily.
If the baby is in the middle of a shed cycle of course, it can still be healthy but may be dull, have cloudy eyes and can even sometimes have slightly wrinkly skin, that's pretty normal for a shedding snake.
