It's good that you've done some research so that you already have an educated eye when you see the snake. Check the temps and the humidity of the enclosure that it's in. Make sure the tank or tub has the husbandry items that balls require. Ask to see the feeding and shedding records.
Look at the snake: Is there any stuck shed? Are their stuck eye caps? Is the skin loose from dehydration or nice and tight? Are the muscles tight and solid? Can you see the ribs as a result from poor weight? Does it curl into a ball when you hold it and then come out of the ball showing some curiosity? What is the vibe you get? (I know, very unscientific, but valid none the less...!)
Let us know what your decision is! Good luck!!